mmc: jz4740_mmc: Fix error check for dma_map_sg

dma_map_sg return 0 on error.

Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20220825074008.33349-3-jinpu.wang@ionos.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>