USB audio gadget: handle endpoint control requests at the function level

Now that control requests targeted at an endpoint can be handled at the
function level, move the UAC-specific control request handling code from
the audio gadget driver to the audio function driver.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>