net: dsa: mv88e6xxx: fix shift of FID bits in mv88e6185_g1_vtu_loadpurge()

[ Upstream commit 48620e341659f6e4b978ec229f6944dabe6df709 ]

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

Fixes: 11ea809f (net: dsa: mv88e6xxx: support 256 databases)
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>