net: dsa: lantiq_gswip: Use dsa_is_cpu_port() in gswip_port_change_mtu()

Make the check for the CPU port in gswip_port_change_mtu() consistent
with other areas of the driver by using dsa_is_cpu_port().

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Link: https://lore.kernel.org/r/20240611135434.3180973-8-ms@dev.tdt.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>