net: bridge: move DSA master bridging restriction to DSA

When DSA gains support for multiple CPU ports in a LAG, it will become
mandatory to monitor the changeupper events for the DSA master.

In fact, there are already some restrictions to be imposed in that area,
namely that a DSA master cannot be a bridge port except in some special
circumstances.

Centralize the restrictions at the level of the DSA layer as a
preliminary step.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>