BugLink: https://launchpad.net/bugs/1512999

Since DMI IDs may be different between models while subsystem IDs of
host bridge will stay the same, use pci_get_subsys() instead of
dmi_match() to identify Edge Gateways.
Signed-off-by: Wen-chien Jesse Sung <jesse.sung@canonical.com>
Acked-by: Brad Figg <brad.figg@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>