iio:adc:max9611: Switch to generic firmware properties.

Note the handling of the device tree node in this driver was somewhat
unusual.  I have cleaned that up whilst also moving over to generic
properties.

Part of a general attempt to move all IIO drivers over to generic
firmware properties both as a general improvement and to avoid sources
of cut and paste into future drivers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>