staging: mt7621-pci: avoid use of global variable 'pcie_link_status'

'pcie_link_status' is a global variable which is being used as a
mask to know which virtual bridges are or not enabled. Instead of
this extract a new 'mt7621_pcie_init_virtual_bridges' function where
a similar local variable is used for the same task. This allow us to
properly configure the virtual PCI-PCI bridges in configuration
registers of the mt7621 pci controller. For a correct initiation of
which is connected use 'enabled' field of 'mt7621_pcie_port' struct.
With this change driver probe function gets a bit cleaner and readable.

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