iio: remove indio_dev pointer from max517_data

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Roland Stigge <stigge@antcom.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>