dmaengine: mmp: fix uninitialized variable

drivers/dma/mmp_tdma.c:236:8: warning: 'tdcr' may be used
uninitialized in this function [-Wuninitialized]

Reported-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>