bridge: make br_device_notifier static

Merge net/bridge/br_notify.c into net/bridge/br.c,
since it has only br_device_event() and br.c is small.

Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>