dmaengine: imx-sdma: fix inconsistent naming in sdma_assign_cookie()

Variable name sdma and sdmac are consistently used as the pointer to
sdma_engine and sdma_channel respectively throughout the file.  The
patch fixes the inconsistency seen in function sdma_assign_cookie().

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>