net: bridge: check vlan with eth_type_vlan() method

Replace some checks for ETH_P_8021Q and ETH_P_8021AD with
eth_type_vlan().

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Link: https://lore.kernel.org/r/20210117080950.122761-1-dong.menglong@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>