dmaengine: tegra20: Split device_control

Split the device_control callback of the NVidia Tegra20 APB DMA 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>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>