net: dsa: mv88e6xxx: get STU entry on VTU GetNext

Now that the code reads both VTU and STU data on VTU GetNext operation,
fetch the STU entry data of a VTU entry at the same time.

The STU data bits are masked with the VTU data bits and they are now all
read at the same time a VTU GetNext operation is issued.

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>