mtd: rawnand: cadence: fix unchecked dereference

Add NULL check before variable dereference to fix static checker warning.

Fixes: d76d22b5 ("mtd: rawnand: cadence: use dma_map_resource for sdma address")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/e448a22c-bada-448d-9167-7af71305130d@stanley.mountain/


Signed-off-by: Niravkumar L Rabara <niravkumar.l.rabara@intel.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>