dmaengine: xilinx: Split device_control

Split the device_control callback of the Xilinx VDMA driver to make use of the
newly introduced callbacks, that will eventually be used to retrieve slave
capabilities.

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>