staging: mt7621-pci: factor out 'mt7621_pcie_enable_port' function

Function 'mt7621_pcie_enable_ports' tries to enable all PCI ports.
To make it more readable the single port initialization part has
been factor out into a new 'mt7621_pcie_enable_port' function.

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