staging:iio: Remove deprecated dev_data from iio_dev.

The equivalent should always be done using iio_alllocate_device
with to create a private area for the driver and then iio_priv
to access it.  There may be other uses for a private data pointer
but right now it just leads driver writers astray.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>