sound: usb: Add return value to debug log

Log handle_uaudio_stream_req() function's return value.
This helps to debug situations where error like -EPROTO
is returned silently if dynamic debug is not enabled in
usb core layer. Also clean up debug log statement.

Change-Id: I9f8821e1d32f210f9a46bdf383dcb7900d4d41bb
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>