staging:iio:adc:ad7150: allocate chip state with iio_dev and use iio_priv to access.

V2 - actually allocate some space for the iio_priv data.

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>