dma: imx-sdma: start transfer in issue_pending

The DMA engine API requires that transfers are started in issue_pending
instead of tx_submit. Fix this.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
[corrected change log to DMA engine API insteadof DMA API]
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>