net: dsa: sja1105: Hide the dsa_8021q VLANs from the bridge fdb command

TX VLANs and RX VLANs are an internal implementation detail of DSA for
frame tagging.  They work by installing special VLANs on switch ports in
the operating modes where no behavior change w.r.t. VLANs can be
observed by the user.

Therefore it makes sense to hide these VLANs in the 'bridge fdb'
command, as well as translate the pvid into the RX VID and TX VID on
'bridge fdb add' and 'bridge fdb del' commands.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>