The suspend/resume functions were using dev_to_iio_dev() to get
the iio_dev. That only works on IIO dev's.  Use dev_get_drvdata()
for a platform device to get the correct iio_dev.
Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>