ALU table entry 2 register in KSZ9477 have bit positions reserved for
forwarding port map. This field is referred in ksz9477_fdb_del() for
clearing forward port map and alu table.

But current fdb_del refer ALU table entry 3 register for accessing forward
port map. Update ksz9477_fdb_del() to get forward port map from correct
alu table entry register.

With this bug, issue can be observed while deleting static MAC entries.
Delete any specific MAC entry using "bridge fdb del" command. This should
clear all the specified MAC entries. But it is observed that entries with
self static alone are retained.

Tested on LAN9370 EVB since ksz9477_fdb_del() is used common across
LAN937x and KSZ series.

Fixes: b987e98e ("dsa: add DSA switch driver for Microchip KSZ9477")
Signed-off-by: Rakesh Sankaranarayanan <rakesh.sankaranarayanan@microchip.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20230118174735.702377-1-rakesh.sankaranarayanan@microchip.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>