mmc: mediatek: enable SDIO IRQ low level trigger function

SDIO IRQ is not defaultly triggered by low level,
but by falling edge. It needs to set related register
to enable SDIO IRQ low level trigger function.
Otherwise the SDIO IRQ may be lost in some specail condition.

Signed-off-by: Yong Mao <yong.mao@mediatek.com>
Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>