mmc: mediatek: Fixed size in dma_free_coherent

The dma gpd dma_free_coherent call size in invalid.

Fixes: 20848903 ("mmc: mediatek: Add Mediatek MMC driver")
Signed-off-by: Phong LE <ple@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>