[BRIDGE]: don't change packet type

The change to forward STP bpdu's (for usermode STP) through normal path,
changed the packet type in the process. Since link local stuff is multicast, it
should stay pkt_type = PACKET_MULTICAST.  The code was probably copy/pasted
incorrectly from the bridge pseudo-device receive path.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>