iio: adc: max1027: Create a helper to enable/disable the cnvst trigger

There are two ways to physically trigger a conversion:
- A falling edge on the cnvst pin
- A write operation on the conversion register

Let's create a helper for this.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210921115408.66711-7-miquel.raynal@bootlin.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>