net: dsa: existing DSA masters cannot join upper interfaces

All the traffic to/from a DSA master is supposed to be distributed among
its DSA switch upper interfaces, so we should not allow other upper
device kinds.

An exception to this is DSA_TAG_PROTO_NONE (switches with no DSA tags),
and in that case it is actually expected to create e.g. VLAN interfaces
on the master. But for those, netdev_uses_dsa(master) returns false, so
the restriction doesn't apply.

The motivation for this change is to allow LAG interfaces of DSA masters
to be DSA masters themselves. We want to restrict the user's degrees of
freedom by 1: the LAG should already have all DSA masters as lowers, and
while lower ports of the LAG can be removed, none can be added after the
fact.

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