After adding support for QinQ, a.k.a 802.1ad protocol, there are a few
scenarios that should be vetoed.

The vetoes are motivated by various ASIC limitations.
For example, a port that is member in a 802.1ad bridge cannot have 802.1q
uppers as the port needs to be configured to treat 802.1q packets as
untagged packets.

Veto all those unsupported scenarios and return suitable messages.
Signed-off-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>