dmaengine: mv_xor: remove redundant null check on cd

The check to see if cd is null is redundant, pdata->channels is
never null at this point, and hence &pdata->channels[i] cannot
be null, so remove the null check.

Detected by CoverityScan, CID#1357194 ("Logically Dead Code")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>