staging: mt7621-mmc: Remove unused static msdc_regs

The static variable msdc_regs is set once, but never used, so remove
it.

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