net: dsa: microchip: ksz8795: Use software untagging on CPU port

commit 9130c2d3 upstream.

On the CPU port, we can support both tagged and untagged VLANs at the
same time by doing any necessary untagging in software rather than
hardware.  To enable that, keep the CPU port's Remove Tag flag cleared
and set the dsa_switch::untag_bridge_pvid flag.

Fixes: e66f840c ("net: dsa: ksz: Add Microchip KSZ8795 DSA driver")
Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backport to 5.10: adjust context]
Signed-off-by: Ben Hutchings <ben.hutchings@mind.be>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>