staging: mt7621-mmc: Change default transfer mode to DMA

The current default transfer is to use DMA or not depending on the
size of the data. The upstream driver mtk-sd uses DMA all times,
change the standard mode here to DMA for testing, if there are any
performance problems with DMA for small data sizes. If not, the option
for transfer mode should be removed in the future,

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