net: dsa: qca8k: Add 802.1q VLAN support

This adds full 802.1q VLAN support to the qca8k, allowing the use of
vlan_filtering and more complicated bridging setups than allowed by
basic port VLAN support.

Tested with a number of untagged ports with separate VLANs and then a
trunk port with all the VLANs tagged on it.

v3:
- Pull QCA8K_PORT_VID_DEF changes into separate cleanup patch
- Reverse Christmas tree notation for variable definitions
- Use untagged instead of tagged for consistency
v2:
- Return sensible errnos on failure rather than -1 (rmk)
- Style cleanups based on Florian's feedback
- Silently allow VLAN 0 as device correctly treats this as no tag

Signed-off-by: Jonathan McDowell <noodles@earth.li>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>