ofproto-dpif-xlate: Identify STP BPDUs more specifically.

Apart from STP, EVB extension of LLDP as well as IEEE 802.1QBG use the
Nearest Customer Bridge (NCB) DMAC which has a value of 0180.c200.0000.
STP can be distinguished by Ethertype from these protocols.

Signed-off-by: Padmanabhan Krishnan <kprad1@yahoo.com>
[blp@nicira.com rewrote the details of the patch]
Signed-off-by: Ben Pfaff <blp@nicira.com>
Tested-by: Padmanabhan Krishnan <kprad1@yahoo.com>