mfd: twl6040: Fix child-node lookup

Fix child-node lookup during probe, which ended up searching the whole
device tree depth-first starting at the parent rather than just matching
on its children.

To make things worse, the parent node was prematurely freed, while the
child node was leaked.

Note that the CONFIG_OF compile guard can be removed as
of_get_child_by_name() provides a !CONFIG_OF implementation which always
fails.

Cc: stable <stable@vger.kernel.org>     # 3.5
Fixes: 37e13cec ("mfd: Add support for Device Tree to twl6040")
Fixes: ca2cad6a ("mfd: Fix twl6040 build failure")
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>