bridge: fix netlink max attr size

.maxtype should match .policy.  Probably just been getting lucky here
because IFLA_BRPORT_MAX > IFLA_BR_MAX.

Fixes: 13323516

 ("bridge: implement rtnl_link_ops->changelink")
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>