net: bridge: log port STP state on change

Remove the shared br_log_state function and print the info directly in
br_set_state, where the net_bridge_port state is actually changed.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>