dmaengine: coh901318: Split device_control

Split the device_control callback of the ST-Ericsson COH901318 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>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>