mmc: mediatek: fix got GPD checksum error interrupt when data transfer

Even if we only use one gpd, we need alloc 2 gpd and make
the gpd->next pointer to the second gpd, or may get gpd checksum
error, this was checked by hardware

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