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

The field cmd_drv of msdc_hw is set to a constant and only used once,
replace that position with the constant and remove the unused field.

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