ASoC: da7218: fix 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 codec node was also prematurely freed.

Fixes: 4d50934a

 ("ASoC: da7218: Add da7218 codec driver")
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable <stable@vger.kernel.org>