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

The xfersz field of msdc_dma is only set to host->xfer_size and this
is also available at the (few) positions, where dma.xfersz is used, so
it is removed.

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