dmaengine: omap-dma: Simplify omap_dma_start_sg parameter list

We can drop the (sg)idx parameter for the omap_dma_start_sg() function and
increment the sgidx inside of the same function.

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