bridge: remove dev_put() in add_del_if()

add_del_if() is called with RTNL, we can use __dev_get_by_index()
instead of [dev_get_by_index() + dev_put()]

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>