dmaengine: Make channel allocation callbacks optional

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: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>