iio: adc: max1027: Introduce an end of conversion helper

For now this helper only waits for the maximum duration of a single
conversion.

In practice, a "temperature measurement" will take twice this
time because it will also carry another analog conversion but as here we
will only care about the temperature conversion which happens first, we
can still only wait for a single sample and get the right data.

This helper will soon be improved to properly handle the end of
conversion interrupt as well as a higher number of samples.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-13-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>