staging: mt7621-mmc: Fix calculation typo in msdc_init_gpd_bd

When refactoring the function msdc_init_gpd_bd, an error in the
calculation of the bd.next address was introduced. The offset has to
be added to the base address, not multiplied.

Fixes: 8f239558 ("staging: mt7621-mmc: Refactor msdc_init_gpd_bd")
Reported-by: NeilBrown <neil@brown.name>
Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>