usb: gadget: f_uac2: Support for multiple channels, sample sizes

Adds multiple alt settings to support multiple channels,
sample sizes. Host can change number of channels and
sample sizes by chosing the appropriate alt setting
for an audio streaming interface.

Change-Id: I1390b70fd3315ed5f9eb5054eb96c6ba318323e3
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>