net: dsa: suppress device links to LAG DSA masters

These don't work (print a harmless error about the operation failing)
and make little sense to have anyway, because when a LAG DSA master goes
away, we will introduce logic to move our CPU port back to the first
physical DSA master. So suppress these device links in preparation for
adding support for LAG DSA masters.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>