usb: audio: Fixed alternate interface request

To pass USBCV chapter 9 tests, alt interface request
must return the last alt interface number set by host.
get_alt function now returns the correct alternate
interface number.

Change-Id: I0d0f7a0c9a9c616dd31395372e7c2bc736e5135d
CRs-Fixed: 436836
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>