mtd: rawnand: cadence: fix DMA device NULL pointer dereference

mtd: rawnand: cadence: fix DMA device NULL pointer dereference

The DMA device pointer `dma_dev` was being dereferenced before ensuring
that `cdns_ctrl->dmac` is properly initialized.

Move the assignment of `dma_dev` after successfully acquiring the DMA
channel to ensure the pointer is valid before use.

Fixes: d76d22b5096c ("mtd: rawnand: cadence: use dma_map_resource for sdma address")
Cc: stable@vger.kernel.org
Signed-off-by: Niravkumar L Rabara <niravkumarlaxmidas.rabara@altera.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>