staging: mt7621-pci: rewrite hardcoded code for enabling ports

There are some hardcoded values in the process for enabling and
disabling some stuff inside 'mt7621_pcie_enable_ports' function.
Create some preprocessor definitions to make code more readable.

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