net: dsa: mv88e6xxx: load STU entry with VTU entry

Now that the code writes both VTU and STU data when loading a VTU entry,
load the corresponding STU entry at the same time.

This allows us to get rid of the STU management in the
_mv88e6xxx_vtu_new helper and thus remove the separate implementations
of STU Load/Purge and STU GetNext, as well as the unused family checks.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>