nvmem: core: Call put_device() in nvmem_unregister()

Call put_device() in nvmem_unregister() to make sure nvmem_release
gets called freeing up allocated resources.

Cc: cphealy@gmail.com
Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>