net: renesas: rswitch: Convert to for_each_available_child_of_node()

Simplify rswitch_get_port_node() by using the
for_each_available_child_of_node() helper instead of manually ignoring
unavailable child nodes, and leaking a reference.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/54f544d573a64b96e01fd00d3481b10806f4d110.1738771798.git.geert+renesas@glider.be


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