net: dsa: mv88e6xxx: assign dynamic FDB to VLANs

Add a _mv88e6xxx_fid_new function which gives and flushes the lowest FID
available. Call it when preparing a new VTU entry.

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