net: dsa: mv88e6xxx: Fix error when setting port policy on mv88e6393x

[ Upstream commit 12bc1494 ]

mv88e6393x_port_set_policy doesn't correctly shift the ptr value when
converting the policy format between the old and new styles, so the
target register ends up with the ptr being written over the data bits.

Shift the pointer to align with the format expected by
mv88e6393x_port_policy_write().

Fixes: 6584b260 ("net: dsa: mv88e6xxx: implement .port_set_policy for Amethyst")
Signed-off-by: Peter Rashleigh <peter@rashleigh.ca>
Reviewed-by: Simon Horman <horms@kernel.org>
Message-ID: <20241016040822.3917-1-peter@rashleigh.ca>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Sasha Levin <sashal@kernel.org>