wcn36xx: fix iris 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 mmio node was also prematurely freed.

Fixes: fd52bdae ("wcn36xx: Disable 5GHz for wcn3620")
Cc: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>