usb: f_uac2: Add SS/SSP support to UAC2 function

Add SS/SSP descriptors for the UAC2 function.

Also fix the audio quality issues by increasing the
number of USB_XFERS.

Change-Id: Ie94aee43dd52b0b711778420e485af72730dd28b
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>
Signed-off-by: Jack Pham <jackp@codeaurora.org>