staging: mt7621-mmc: Don't disables DMA

Current code disables the DMA after the transfer. That only changes
the transfer mode to non-DMA and does not save any power. This is not
necessary any more, because now DMA transfer is always used. The macro
for disabling DMA transfer is also removed, because it is not used any
more.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>