ramips: mtk-mmc: don't advertise highmem support.

ramips: mtk-mmc: don't advertise highmem support.

of_dma_configure() sets a default ->dma_mask of
DMA_BIT_MASK(32), claiming devices can DMA from
the full 32bit address space.
The mtk-mmc driver does not support access to
highmem pages, so it is really limited to the
bottom 512M (actually 448M due to 64M of IO space).
Setting ->dma_mask to NULL causes mmc_setup_queue()
to fall-back to using BLK_BOUNCE_HIGH to tell the
block layer to use a bounce-buffer for any highmem
pages requiring IO.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Felix Fietkau <nbd@nbd.name>