usb: dwc3: rtk: Clean up error code in __get_dwc3_maximum_speed()

The __get_dwc3_maximum_speed() function returns an enum type which, in
this context here, is basically unsigned int.  On error cases, it's
supposed to return USB_SPEED_UNKNOWN, but it was accidentally changed to
return negative error codes in commit f93e96c ("usb: dwc3: rtk: use
scoped device node handling to simplify error paths").

There is only one caller and because of the way that the types work out,
returning negative error codes is not a problem.  They will be treated
as greater than USB_SPEED_HIGH and ignored as invalid.  So this patch
does not affect run time behavior, it's just a clean up.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/865e56dc-37cc-47b1-8d35-9047ecb1984a@stanley.mountain
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Dan Carpenter

drivers/usb/dwc3/dwc3-rtk.c

@@ -184,7 +184,7 @@ static enum usb_device_speed __get_dwc3_maximum_speed(struct device_node *np)
 
 	ret = of_property_read_string(dwc3_np, "maximum-speed", &maximum_speed);
 	if (ret < 0)
-		return ret;
+		return USB_SPEED_UNKNOWN;
 
 	ret = match_string(speed_names, ARRAY_SIZE(speed_names), maximum_speed);