phy: tegra: fix device-tree node lookups

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

To make things worse, some parent nodes could end up being being
prematurely freed (by tegra_xusb_pad_register()) as
of_find_node_by_name() drops a reference to its first argument.

Fixes: 53d2a715 ("phy: Add Tegra XUSB pad controller support")
Cc: stable <stable@vger.kernel.org>     # 4.7
Cc: Thierry Reding <treding@nvidia.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>