net: dsa: mv88e6xxx: add VLAN support to FDB dump

Add an helper function to read the next valid VLAN entry for a given
port. It is used in the VID to FID conversion function to retrieve the
forwarding database assigned to a given VLAN port.

Finally update the FDB getnext operation to iterate on the next valid
port VLAN when the end of the current database is reached.

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