net: dsa: mv88e6xxx: simplify VTU entry getter

Make the code which fetches or initializes a new VTU entry more concise.
This allows us the get rid of the old underscore prefix naming.

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>