staging: mt7621-mmc: Remove code guarded by nonexistent config option

Remove a piece of code that is guarded by a config option, that is
nonexistent, the function it calls is also nonexistent.

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