usb: gadget: uac2: add req_number as parameter

There are only two requests for uac2, it may not be enough at high
loading system which usb interrupt handler can't be serviced on
time, then the data will be lost since it is isoc transfer for audio.

In this patch, we introduce a parameter for the number for usb request,
and the user can override it if current number for request is not enough
for his/her use case.

Besides, update this parameter for legacy audio gadget and documentation.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/usb/gadget-testing.txt

@@ -632,6 +632,8 @@ The uac2 function provides these attributes in its function directory:
 	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
 
 The attributes have sane default values.