usb:gadget:audio_source: Move to USB_FUNCTION API

This patch adds support to use audio_source
gadget function through DECLARE_USB_FUNCTION_INIT
interface.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: I1fc6c9ea07105ae4eb785eebd3bb925bfdd8bc6b