usb: gadget: audio: Always write fixed buff_size data to Audio

Audio driver expects 'audio_playback_buf_size' data in each
playback command. If USB received fewer data from host PC and
gives same to PCM lib then Audio driver still fetches
'audio_playback_buf_size' data resulting in sending over junk
or repeated uplink data to AFE. While at it also fix the audio
playback and capture device names.

Change-Id: Icd600e27a513c5d44e6a2703d60cb3fcd03ae5fe
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>