dma: mxs-dma: enable channel in device_issue_pending call

Enable channel in device_issue_pending call, so that the order between
cookie assignment and channel enabling can be ensured naturally.

It fixes the mxs gpmi-nand breakage which is caused by the incorrect
order of cookie assigning and channel enabling.

Suggested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Tested-by: Huang Shijie <b32955@freescale.com>
Tested-by <samgandhi9@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>