iio: adc: max1118: Avoid jumping back and forth between spi and iio structures

Changing from passing the spi structure into various functions to
passing struct iio_dev avoids use of spi_get_drvdata and lets us
stop setting that at all.  Previous code was unnecessarily complex.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Alexandru Ardelean <aardelean@deviqon.com>
Link: https://lore.kernel.org/r/20210516172520.1398835-5-jic23@kernel.org