staging: mt7621-mmc: Remove unused field sg from msdc_dma

The field sg from struct msdc_dma is once set and never read, so
remove it.

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