Previous version of ad7091r event handler received the ADC state pointer
and retrieved the iio device from driver data field with dev_get_drvdata().
However, no driver data have ever been set, which led to null pointer
dereference when running the event handler.

Pass the iio device to the event handler and retrieve the ADC state struct
from it so we avoid the null pointer dereference and save the driver from
filling the driver data field.

Fixes: ca693001 ("iio: adc: Add support for AD7091R5 ADC")
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Link: https://lore.kernel.org/r/5024b764107463de9578d5b3b0a3d5678e307b1a.1702746240.git.marcelo.schmitt1@gmail.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>