mtd: rawnand: remove unecessary checking if dmac is NULL

Remove unecessary checking if dmac is NULL.

If Cadence nand controller driver uses DMA engine then cdns_ctrl->dmac
cannot be NULL. It is verified during driver initialization.
If Cadence nand controller driver does not use DMA engine then
CPU IO read/write are executed instead of slave DMA transfer.
In that case cdns_ctrl->dmac is not used at all.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Piotr Sroka <piotrs@cadence.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>