nvmem: add missing of_node_put() in of_nvmem_cell_get()

of_get_next_parent() increments the refcount of the returned node.
It should be put when done.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>