dmaengine: fsl-dma: Split device_control

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

While we're at it, remove the useless prep_sg callback.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>