iio: health: afe4404: retrieve a valid iio_dev in suspend/resume

The suspend/resume functions were using dev_to_iio_dev() to get
the iio_dev. That only works on IIO dev's.  Replace it with i2c
functions to get the correct iio_dev.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>