mtd: rawnand: rockchip: Add missing check after DMA map

The DMA map functions can fail and should be tested for errors.

Fixes: 058e0e84 ("mtd: rawnand: rockchip: NFC driver for RK3308, RK2928 and others")
Signed-off-by: Thomas Fourier <fourier.thomas@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>