net: dsa: lantiq_gswip: Remove dead code from gswip_add_single_port_br()

The port validation in gswip_add_single_port_br() is superfluous and
can be omitted.

Suggested-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Martin Schiller <ms@dev.tdt.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20240611135434.3180973-11-ms@dev.tdt.de


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