net: dsa: lan9303: Fix lan9303_alr_del_port()

Fix embarrassing bug in lan9303_alr_del_port(): Instead of zeroing
entr->mac_addr, I destroyed the next cache entry. Affected .port_fdb_del and
.port_mdb_del.

Fixes: 0620427e

 ("net: dsa: lan9303: Add fdb/mdb manipulation")
Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>