staging: mt7621-mmc: Remove unused variable dma_xfer from msdc_host

The field dma_xfer of the struct msdc_host is not used anymore, remove
it.

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