dmaengine: imx-dma: remove unused arg of imxdma_sg_next.

Since this function is always used with 'desc' as first argument
and 'desc->sg' as second argument, the latter is clearly redundant
and can be removed.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>