USB audio gadget: Un-inline generic_[gs]et_cmd

Those functions are used only used to fill the set/get members of
usb_audio_control. It doesn't make much sense to inline them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@skynet.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>