usb: gadget: f_audio_source: Fixed USB Audio Class Interface Descriptor

Fixed Android Issue #56549.

When both Vendor Class and Audio Class are activated for AOA 2.0,
the baInterfaceNr of the AudioControl Interface Descriptor points
to wrong interface numbers. They should be pointing to
Audio Control Device and Audio Streaming interfaces.

Replaced baInterfaceNr with the correct value.

Change-Id: Ib20f20c1d65887bac051b7ff9453dc53fc23a8d0
Signed-off-by: Anson Jacob <ansonkuzhumbil@gmail.com>