staging: mt7621-pci: use rt_sysc_?32() to access system-control register.

This driver currently has internal knowledge ofthe address
of system-control registers and accesses them by dereferencing
a constant pointer.

It is cleaner to use rt_sysc_r32(), rt_sysc_w32(), rt_sysc_m32()
which is a more standard interface.
So change the defined names to offsets instead of pointers,
and use these functions.

Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>