net: dsa: felix: improve the workaround for multiple native VLANs on NPI port

After the good discussion with Florian from here:
https://lore.kernel.org/netdev/20200911000337.htwr366ng3nc3a7d@skbuf/

I realized that the VLAN settings on the NPI port (the hardware "CPU port",
in DSA parlance) don't actually make any difference, because that port
is hardcoded in hardware to use what mv88e6xxx would call "unmodified"
egress policy for VLANs.

So earlier patch 183be6f9

 ("net: dsa: felix: send VLANs on CPU port
as egress-tagged") was incorrect in the sense that it didn't actually
make the VLANs be sent on the NPI port as egress-tagged. It only made
ocelot_port_set_native_vlan shut up.

Now that we have moved the check from ocelot_port_set_native_vlan to
ocelot_vlan_prepare, we can simply shunt ocelot_vlan_prepare from DSA,
and avoid calling it. This is the correct way to deal with things,
because the NPI port configuration is DSA-specific, so the ocelot switch
library should not have the check for multiple native VLANs refined in
any way, it is correct the way it is.

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