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

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

Fixes: 764f1b748164 ("spi: add driver for MTK SPI NAND Flash Interface")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Chuanhong Guo <[e-mail]>
Link: https://lore.kernel.org/r/YmwjUcTKyQNrrn2g@kili
Signed-off-by: Mark Brown <[e-mail]>

drivers/spi/spi-mtk-snfi.c

···
 	nfi_write32(snf, NFI_CON, (snf->nfi_cfg.nsectors << CON_SEC_NUM_S));
 
 	buf_dma = dma_map_single(snf->dev, buf, dma_len, DMA_FROM_DEVICE);
-	if (dma_mapping_error(snf->dev, buf_dma)) {
+	ret = dma_mapping_error(snf->dev, buf_dma);
+	if (ret) {
 		dev_err(snf->dev, "DMA mapping failed.\n");
 		goto cleanup;
 	}
···
 
 	nfi_write32(snf, NFI_CON, (snf->nfi_cfg.nsectors << CON_SEC_NUM_S));
 	buf_dma = dma_map_single(snf->dev, snf->buf, dma_len, DMA_TO_DEVICE);
-	if (dma_mapping_error(snf->dev, buf_dma)) {
+	ret = dma_mapping_error(snf->dev, buf_dma);
+	if (ret) {
 		dev_err(snf->dev, "DMA mapping failed.\n");
 		goto cleanup;
 	}