net: dsa: mv88e6xxx: fix shift of FID bits in mv88e6250_g1_vtu_loadpurge()

The comment is correct, but the code ends up moving the bits four
places too far, into the VTUOp field.

Fixes: bec8e572

 (net: dsa: mv88e6xxx: implement vtu_getnext and vtu_loadpurge for mv88e6250)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>