Nowadays, some drivers don't have anything in there channel allocation
callbacks anymore.

Remove the BUG_ON if those callbacks aren't implemented, in order to allow
drivers to not implement them.
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>