usb: gadget: audio: Queue multiple buffers for TX (capture)

Driver is currently using only one buffer for TX. This may
cause underruns at USB hardware if interrupt threshold is
greater than or equal to bInterval. Use multiple buffers (4)
to handle this and also reduce RX buffers from 256 to 48,
i.e. ability to handle 48msec of system latency which
should be sufficient.

Change-Id: I9758aef3e88e066da28e7eb2cbc5b8c6f56d47c1
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>