ANDROID: usb: gadget: f_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

[AmitP: Folded following android-4.9 commit changes into this patch
        95106e3d2071 ("ANDROID: usb: gadget: build audio_source function only if SND is enabled")
        4de9e33e5046 ("ANDROID: usb: gadget: audio_source: fix comparison of distinct pointer types")
        Parts of 051584e76d12 ("ANDROID: usb: gadget: function: cleanup: Add blank line after declaration")]
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>