net: dsa: microchip: ksz_common: Fix refcount leak bug

In ksz_switch_register(), we should call of_node_put() for the
reference returned by of_get_child_by_name() which has increased
the refcount.

Fixes: 912aae27

 ("net: dsa: microchip: really look for phy-mode in port nodes")
Signed-off-by: Liang He <windhl@126.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20220714153138.375919-1-windhl@126.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>