leds: lm36274: don't iterate through children since there is only one

Do not use device_for_each_child_node. Since this driver works only with
once child node present, use device_get_next_child_node instead.
This also saves one level of indentation.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>