staging: mt7621-mmc: Remove non-DMA transfer code

Currently the driver is capable of DMA and non-DMA transfer. But the
option to choose non-DMA transfer has already been removed. Now remove
also the code for the non-DMA transfer.

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