powerpc/spufs: Fix refcount leak in spufs_init_isolated_loader

of_find_node_by_path() returns remote device nodepointer with
refcount incremented, we should use of_node_put() on it when done.
Add missing of_node_put() to avoid refcount leak.

Fixes: 0afacde3 ("[POWERPC] spufs: allow isolated mode apps by starting the SPE loader")
Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220603121543.22884-1-linmq006@gmail.com