net: dsa: mv88e6xxx: do not skip ports on VLAN del

The mv88e6xxx driver currently tries to be smart and remove by itself a
VLAN entry from the VTU when the driven switch sees no user ports as
members of the VLAN.

This is bad in a multi-chip switch fabric, since a chip in between
others may have no bridge port members, but still needs to be aware of
the VID in order to correctly pass frames in the data path.

Now that the DSA core explicitly manages DSA and CPU ports, do not skip
them when checking remaining VLAN members.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>