dmaengine: img-mdc: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later at line 850:
mdma->soc = match->data;

For fixing this problem, we use of_device_get_match_data(), this will
simplify the code a little by using a standard function for
getting the match data.

This was reported by coverity (CID 1324134)

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>