bcm2835-i2s: add 24bit support, update bclk_ratio to more correct values

Code ported from bcm2708-i2s driver in Raspberry Pi tree.

RPi commit 62c05a0 ("ASoC: BCM2708:
Add 24 bit support")

This adds 24 bit support to the I2S driver of the BCM2708.
Besides enabling the 24 bit flags, it includes two bug fixes:

MMAP is not supported. Claiming this leads to strange issues
when the format of driver and file do not match.

The datasheet states that the width extension bit should be set
for widths greater than 24, but greater or equal would be correct.
This follows from the definition of the width field.

Signed-off-by: Florian Meier <florian.meier@koalo.de>

RPi commit 3e8c672 ("bcm2708-i2s:
Update bclk_ratio to more correct values")

Discussion about blck_ratio affecting sound quality:
#681

Signed-off-by: Matthias Reichl <hias@horus.com>

sound/soc/bcm/bcm2835-i2s.c

@@ -340,11 +340,15 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
 	switch (params_format(params)) {
 	case SNDRV_PCM_FORMAT_S16_LE:
 		data_length = 16;
-		bclk_ratio = 40;
+		bclk_ratio = 50;
+		break;
+	case SNDRV_PCM_FORMAT_S24_LE:
+		data_length = 24;
+		bclk_ratio = 50;
 		break;
 	case SNDRV_PCM_FORMAT_S32_LE:
 		data_length = 32;
-		bclk_ratio = 80;
+		bclk_ratio = 100;
 		break;
 	default:
 		return -EINVAL;
@@ -420,7 +424,7 @@ static int bcm2835_i2s_hw_params(struct snd_pcm_substream *substream,
 	/* Setup the frame format */
 	format = BCM2835_I2S_CHEN;
 
-	if (data_length > 24)
+	if (data_length >= 24)
 		format |= BCM2835_I2S_CHWEX;
 
 	format |= BCM2835_I2S_CHWID((data_length-8)&0xf);
@@ -711,13 +715,15 @@ static struct snd_soc_dai_driver bcm2835_i2s_dai = {
 		.channels_max = 2,
 		.rates =	SNDRV_PCM_RATE_8000_192000,
 		.formats =	SNDRV_PCM_FMTBIT_S16_LE
+				| SNDRV_PCM_FMTBIT_S24_LE
 				| SNDRV_PCM_FMTBIT_S32_LE
 		},
 	.capture = {
 		.channels_min = 2,
 		.channels_max = 2,
 		.rates =	SNDRV_PCM_RATE_8000_192000,
 		.formats =	SNDRV_PCM_FMTBIT_S16_LE
+				| SNDRV_PCM_FMTBIT_S24_LE
 				| SNDRV_PCM_FMTBIT_S32_LE
 		},
 	.ops = &bcm2835_i2s_dai_ops,