usb: audio: Fixed double scheduling of playback_work issue

If audio function recevied set_alt requests at a specified
order an issue would happen when the same playback buffer
would be scheduled for playback twice and that would lead
to a crash. This fix prevents this issue by insuring no
buffer is scheduled more than once.

Change-Id: If2a0a7262e6861c084173b9aa956abe840fe3e20
CRs-Fixed: 436836
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>