usb: gadget: f_uac1: add volume and mute support

This adds bi-directional (host->device, device->host)
volume/mute support to the f_uac1 driver by adding
Feature Units and interrupt endpoint.

Currently only master channel is supported.

Volume and mute are configurable through configfs,
by default volume has -100..0 dB range with 1 dB step.

Similar to existing flexible endpoints configuration,
Feature Unit won't be added to the topology if both
mute and volume are not enabled, also interrupt endpoint
isn't added to the device if no feature unit is present

Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>
Signed-off-by: Pavel Hofman <pavel.hofman@ivitera.com>
Link: https://lore.kernel.org/r/20210712125529.76070-5-pavel.hofman@ivitera.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/configfs-usb-gadget-uac1

@@ -8,9 +8,19 @@ Description:
 		c_chmask	capture channel mask
 		c_srate		capture sampling rate
 		c_ssize		capture sample size (bytes)
+		c_mute_present	capture mute control enable
+		c_volume_present	capture volume control enable
+		c_volume_min	capture volume control min value (in 1/256 dB)
+		c_volume_max	capture volume control max value (in 1/256 dB)
+		c_volume_res	capture volume control resolution (in 1/256 dB)
 		p_chmask	playback channel mask
 		p_srate		playback sampling rate
 		p_ssize		playback sample size (bytes)
+		p_mute_present	playback mute control enable
+		p_volume_present	playback volume control enable
+		p_volume_min	playback volume control min value (in 1/256 dB)
+		p_volume_max	playback volume control max value (in 1/256 dB)
+		p_volume_res	playback volume control resolution (in 1/256 dB)
 		req_number	the number of pre-allocated request
 				for both capture and playback
 		==========	===================================

Documentation/usb/gadget-testing.rst

@@ -915,14 +915,24 @@ The function name to use when creating the function directory is "uac1".
 The uac1 function provides these attributes in its function directory:
 
 	========== ====================================================
-	c_chmask   capture channel mask
-	c_srate    capture sampling rate
-	c_ssize    capture sample size (bytes)
-	p_chmask   playback channel mask
-	p_srate    playback sampling rate
-	p_ssize    playback sample size (bytes)
-	req_number the number of pre-allocated request for both capture
-		   and playback
+	c_chmask   	capture channel mask
+	c_srate    	capture sampling rate
+	c_ssize    	capture sample size (bytes)
+	c_mute_present	capture mute control enable
+	c_volume_present	capture volume control enable
+	c_volume_min	capture volume control min value (in 1/256 dB)
+	c_volume_max	capture volume control max value (in 1/256 dB)
+	c_volume_res	capture volume control resolution (in 1/256 dB)
+	p_chmask   	playback channel mask
+	p_srate    	playback sampling rate
+	p_ssize    	playback sample size (bytes)
+	p_mute_present	playback mute control enable
+	p_volume_present	playback volume control enable
+	p_volume_min	playback volume control min value (in 1/256 dB)
+	p_volume_max	playback volume control max value (in 1/256 dB)
+	p_volume_res	playback volume control resolution (in 1/256 dB)
+	req_number 	the number of pre-allocated request for both capture
+			and playback
 	========== ====================================================
 
 The attributes have sane default values.