staging: mt7621-pci: remap and use sysctl from device tree

There are some pointer read and writes which can be replaced
properly using sysctl registers readed from device tree. Remap
sysctl registers and replace in proper places.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>