dmaengine: Remove device_control and device_slave_caps

Now that device_control has been split into several functions, and
device_slave_caps rendered useless, we can safely remove 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>