Use the new iio_trigger_validate_own_device() to verify that the trigger
can only be attached to the matching IIO device rather than using a custom
variant.

While the implementation of iio_trigger_validate_own_device() and the
custom variant and are not identical their behaviour is.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>