net: dsa: mv88e6xxx: extract FID write from ATU command

Not every ATU commands apply to an FID, thus remove the FID writing from
mv88e6xxx_atu_cmd and write it explicitly where needed, in order to ease
introduction of such commands.

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