dmaengine: omap-dma: Use pointer to omap_sg in slave_sg setup's loop

Instead of accessing the array via index, take the pointer first and use
it to set up the omap_sg struct.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>