staging:iio:adc:ad7476 allocate state with iio_dev and use iio_priv to access.

Reg handling is a little fiddly given the ordering of calls.

All part of getting rid of iio_dev->dev_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>