iio: adc: max1027: Ensure a default cnvst trigger configuration

iio: adc: max1027: Ensure a default cnvst trigger configuration

We don't expect the (hardware) cnvst trigger to be enabled at boot time,
this is a user choice made in sysfs and there is a dedicated callback to
enable/disable this trigger. Hence, we can just ensure it is disabled in
the probe at initialization time and then assume that whenever a
->read_raw() call happens, the trigger has been disabled and conversions
will start on register write.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-9-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>