mtd: rawnand: lpx32xx: Fix dma_request_chan() error checks

The dma_request_chan() returns error pointer in case of error, while
dma_request_channel() returns NULL in case of error therefore different
error checks are needed for the two.

Fixes: 7326d3fb1ee3 ("mtd: rawnand: lpx32xx: Request DMA channels using DT entries")
Signed-off-by: Piotr Wojtaszczyk <piotr.wojtaszczyk@timesys.com>
Cc: stable@vger.kernel.org
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240705115139.126522-1-piotr.wojtaszczyk@timesys.com