iio: adc: max1027: Separate the IRQ handler from the read logic

Create a max1027_read_scan() helper which will make clearer the future IRQ
handler updates (no functional change).

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-12-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>