nvmem: use list_for_each_entry_safe in nvmem_device_remove_all_cells()

Use the provided helper for iterating over list entries without having
to use the list_entry() macro.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>