staging: mt7621-mmc: Remove unused field disable_cd_eirq

The disable_cd_eirq() field of msdc_hw is never set and only once
checked for being non zero, so it is deleted.

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