staging: mt7621-mmc: Remove transfer mode setting from proc

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.

There is no apparent reason why somebody would like to change the
transfer mode and the position of the setting in the debug part of the
driver also indicates, that the option was used for debugging
purposes. So it is removed to clean up the driver and bring it more in
line with the upstream one.

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