iio: adc: max1027: Allow all kind of triggers to be used

There is no reason to limit this driver to its internal trigger. The
only difference being, when using an external trigger, the sample
conversion must be manually started.

Drop the ->validate_trigger() hook in order to allow other triggers to
be bound.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20210921115408.66711-16-miquel.raynal@bootlin.com


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