net: dsa: mv88e6xxx: move STU GetNext operation

Extract the generic portion of code to issue an STU GetNext operation,
which will be used in other implementations.

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>