USB: f_uac1: add iad descriptor for UAC1 driver

Add iad descriptor to UAC1 driver to publish it as part of
descriptors to host during enumeration. Else audio over usb
may not work with some hosts if UAC1 and UVC is present in
same composition.

Change-Id: I62f224a9890a846e1e66a9612607b9bde8bed503
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>