mtd: nand: realtek-ecc: Fix a IS_ERR() vs NULL bug in probe

mtd: nand: realtek-ecc: Fix a IS_ERR() vs NULL bug in probe

The dma_alloc_noncoherent() function doesn't return error pointers, it
returns NULL on error.  Fix the error checking to match.

Fixes: 3148d0e5b1c5 ("mtd: nand: realtek-ecc: Add Realtek external ECC engine support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>