staging: mt7621-mmc: Remove unused fields from msdc_dma

The fields used_gpd|bd in struct msdc_dma are only set and reset, but
never used, so remove them. The other removed fields are never used,
so they were removed either.

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