staging: mt7621-pci: add two helpers for read and write pcie register ports

mt7621-pcie_port data structure has filed 'base' as the base address for
read and write related port registers. Create two inline functions
'pcie_port_read' and 'pcie_port_write' to make this task easier and
code more readable.

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