bridge: Fix compilation warning in function br_stp_recalculate_bridge_id()

net/bridge/br_stp_if.c: In function ‘br_stp_recalculate_bridge_id’:
net/bridge/br_stp_if.c:216:3: warning: ‘return’ with no value, in function returning non-void

Signed-off-by: G.Balaji <balajig81@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>