net: mscc: ocelot: deny changing the native VLAN from the prepare phase

Put the preparation phase of switchdev VLAN objects to some good use,
and move the check we already had, for preventing the existence of more
than one egress-untagged VLAN per port, to the preparation phase of the
addition.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>