net: dsa: lantiq_gswip: Fix start index in gswip_port_fdb()

The first N entries in priv->vlans are reserved for managing ports which
are not part of a bridge. Use priv->hw_info->max_ports to consistently
access per-bridge entries at index 7. Starting at
priv->hw_info->cpu_port (6) is harmless in this case because
priv->vlan[6].bridge is always NULL so the comparison result is always
false (which results in this entry being skipped).

Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>