staging: mt7621-pci: factor out 'mt7621_pcie_enable_ports' function

Driver probe function check for a bit in 'pcie_link_status' for
enabling the pcie port or not. Instead of this add 'enabled' field
in 'mt7621_pcie_port' structure and check its value to achieve this
inside a new 'mt7621_pcie_enable_ports' function. The offsets for
the correct port are derived from the slot of the port. This field
'enabled' is properly initilized in 'mt7621_pcie_init_port'. Also
depending of the number of ports enabled we have to properly init
its registers taking into account that we will use virtual bridges
from zero to num_slots_enabled as follows:
    - Only one enabled -> only enable virtual bridge 0.
    - Two enabled -> enable virtual bridges 0 and 1.
    - Three enabled -> enable virtual bridges 0, 1 and 2.

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