staging: mt7621-mmc: Remove transfer mode setting variable

Currently the transfer mode can be chosen between DMA, a size
dependent mode and non-DMA by writing to a proc file. The upstream
driver mtk-sd uses DMA all times. The previous patch removed the
ability to set that option.

Now the remaining uses of the transfer mode setting variable are
cleaned up, because it cannot be changed any more.

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