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: Iaa083f3d97c1f0fc9481bf87852b2b51278a6351
Signed-off-by: Anson Jacob <ansonkuzhumbil@gmail.com>