net: dsa: check bridge VLAN in slave operations

The bridge VLANs are not offloaded by dsa_port_vlan_* if the port is
not bridged or if its bridge is not VLAN aware.

This is a good thing but other corners of DSA, such as the tag_8021q
driver, may need to program VLANs regardless the bridge state.

And also because bridge_dev is specific to user ports anyway, move
these checks were it belongs, one layer up in the slave code.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>