soc/tegra: Don't leak device tree node reference

of_find_node_by_path() acquires a reference to the node returned by it
and that reference needs to be dropped by its caller. soc_is_tegra()
doesn't do that, so fix it.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
[treding: slightly rewrite to avoid inline comparison]
Signed-off-by: Thierry Reding <treding@nvidia.com>