staging: mt7621-pci: avoid mapping sysctls registers

The sysctl register are already claimed by palmbus, so
pci fails to claim it. The best way to access the sysc registers
is to use rt_sysc_[rwm]32().

Fixes: 89e9f6e6: staging: mt7621-pci: remap and use sysctl from device tree
Reported-by: NeilBrown <neil@brown.name>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Tested-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>