staging: mt7621-mmc: Remove unused field enable_cd_eirq from msdc_hw

The enable_cd_eirq() field of msdc_hw is never set and only once
checked for not being zero, so it is removed.

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