dmaengine: omap-dma: move barrier to omap_dma_start_desc()

We don't need to issue a barrier for every segment of a DMA transfer;
doing this just once per descriptor will do.

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>