staging: mt7621-pci: simplify 'mt7621_pcie_init_virtual_bridges' function

staging: mt7621-pci: simplify 'mt7621_pcie_init_virtual_bridges' function

Function 'mt7621_pcie_init_virtual_bridges' is a bit mess and can be
refactorized properly in a cleaner way. Introduce new 'pcie_rmw' inline
function helper to do clear and set the correct bits this function needs
to work.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200308091928.17177-1-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>