stp: Make stp-disabled port forward stp bpdu packets.

Commit 0d1cee123a84 (stp: Fix bpdu tx problem in listening state)
makes ovs drop the stp bpdu packets if stp is not enabled on the
input port.  However, when pif bridge is used and stp is enabled
on the integration bridge.  The flow translation of stp bpdu
packets will go through a level of resubmission which changes
the input port to the corresponding peer port.  Since, the
patch port on the pif bridge does not have stp enabled, the
flow translation will drop the bpdu packets.

This commit fixes the issue by making ovs forward stp bpdu packets
on stp-disabled port.

VMware-BZ: #1284695

Signed-off-by: Alex Wang <alexw@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>