staging: mt7621-mmc: set correct dma mask.

Since commit f8c55dc6

 ("MIPS: use generic dma noncoherent ops for
simple noncoherent platforms") changed MIPS dma handling, the mmc
driver fails because it doesn't have a dma mask is set.

So set the correct dma mask.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>