staging: mt7621-pci: factor out 'mt7621_enable_phy' function

Factor out a new function 'mt7621_enable_phy' for enabling the
pcie phy for each port and call it from 'mt7621_pcie_enable_port'.

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