staging: iio: call iio_device_unregister() in max517_remove()

Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>