staging: mt7621-pci: avoid register duplication per controller using pcie_[read|write]

Use pcie_[read|write] fucntions to read and write controller registers.
Define those only by offset and pass controller offset + register offset
relative to base address to functions.

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