staging: mt7621-pci: factor out 'mt7621_pcie_init_ports' function

Instead of just inline this code in the probe driver function factor
out a new 'mt7621_pcie_init_ports' function to achieve this.

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