regulator: ltc3589: Use of_get_child_by_name

of_find_node_by_name() walks the allnodes list, and can thus walk
outside of the parent node. Use of_get_child_by_name() instead.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>