staging: mt7621-pci: make use of 'pcie_port_write'

staging: mt7621-pci: make use of 'pcie_port_write'

Function 'mt7621_pcie_enable_port' is calculating an offset
to write some port related registers. Instead of doing that
just make use of already existent 'pcie_write_port' function
and use virtualy mapped base address with registers. This
increase readability and allow us to remove also two defitions
not used else where.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210607120153.24989-7-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>