soc/tegra: pmc: Avoid usage of uninitialized variable

Make sure to only drop the reference to the OF node after it's been
successfully obtained.

Fixes: 3568df3d

 ("soc: tegra: Add thermal reset (thermtrip) support to PMC")
Cc: <stable@vger.kernel.org> # v4.0+
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>