The STU (if the switch has one) is abstracted and accessed through the
VTU operations and data registers.

Thus rename the mv88e6xxx_vtu_stu_entry struct to mv88e6xxx_vtu_entry.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>