[BRIDGE]: Fix deadlock in br_stp_disable_bridge

Looks like somebody forgot to use the _bh spin_lock variant. We ran into a 
deadlock where br->hello_timer expired while br_stp_disable_br() walked 
br->port_list. 

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