dmaengine: omap-dma: consolidate setup of CSDP

Consolidate the setup of the channel source destination parameters
register.  This way, we calculate the required CSDP value when we setup
a transfer descriptor, and only write it to the device registers once
when we start the descriptor.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>