staging:iio:resolver:ad2s90 fix registration of null pointer

When the new allocation code was introduced a stray pointer
to iio_dev structure was left in the chip state structure.
This was never set but was then registered with the core.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>