staging: mt7621-mmc: Remove code for different soc types

Remove code that is intended for soc other then the mt7621 because
this driver is only for that soc. This also removes some conditions.
However the remaining code is also at the wrong place, it just selects
the mode of the pins, this is a job for pinctl not for the sdxc driver.

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