staging: mt7621-mmc: Fix dma_map_sg may map to fewer entries

The dma_map_sg function may merge several sglist entries into one, the
return value has to be saved to consider that. The data->sg_count
field is the position, where it should be saved, like it is done in
other mmc host drivers. Also the count of mapped entries is needed for
the dma setup.

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