[BRIDGE]: random extra bytes on STP TCN packet

We seem to send 3 extra bytes in a TCN, which will be whatever happens
to be on the stack. Thanks to Aji_Srinivas@emc.com for seeing.

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