usb: gadget: f_uac2: convert to new function interface with backward compatibility

Converting uac2 to the new function interface requires converting
the USB uac2's function code and its users.

This patch converts the f_uac2.c to the new function interface.

The file is now compiled into a separate usb_f_uac2.ko module.

The old function interface is provided by means of a preprocessor
conditional directives. After all users are converted, the old interface
can be removed.

Tested-by: Sebastian Reimers <sebastian.reimers@googlemail.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/legacy/audio.c

@@ -45,6 +45,7 @@ static struct usb_gadget_strings *audio_strings[] = {
 #include "u_uac1.c"
 #include "f_uac1.c"
 #else
+#define USB_FUAC2_INCLUDED
 #include "f_uac2.c"
 #endif