staging: mt7621-mmc: Remove power callbacks from msdc_hw

The ext_power_on|off fields of msdc_hw are never set, and only once
checked for not being zero, so they can just be removed.

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