mmc: meson-mx-sdhc: Fix error check for dma_map_sg

mmc: meson-mx-sdhc: Fix error check for dma_map_sg

dma_map_sg return 0 on error, also change the type for dma_len
from int to unsigned int.

Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Jerome Brunet <jbrunet@baylibre.com>
Cc: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jack Wang <jinpu.wang@ionos.com>
Link: https://lore.kernel.org/r/20220825074008.33349-2-jinpu.wang@ionos.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>