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