staging: mt7621-mmc: Remove constant flag MSDC_HIGHSPEED

The MSDC_HIGHSPEED flag is always set and never unset, remove it to
clean up the code.

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