spi: mtk-snfi: preserve dma_mapping_error() error codes

Return -ENOMEM of there is a dma mapping error.  Do not return success.

Fixes: 764f1b74 ("spi: add driver for MTK SPI NAND Flash Interface")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Chuanhong Guo <gch981213@gmail.com>
Link: https://lore.kernel.org/r/YmwjUcTKyQNrrn2g@kili


Signed-off-by: Mark Brown <broonie@kernel.org>