sound: usb: Add helper APIs to enable audio stream

Adding helper API to find usb substream context information
using card number, pcm device number and direction. This usb
substream is used to enable/disable by issuing SET_ALT
command to device. Also add disconnect call back to perform
any clean up required.

Change-Id: I252f5171bd94b5ab096eb1a2f053f29a8c049c3b
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>