iio: adis_trigger: Remove code to set trigger parent

iio_trigger_set_drvdata() sets the trigger device parent to first
argument of viio_trigger_alloc(), no need to do it again in the driver
code.
Remove adis_trigger_setup() to match other drivers where setting the
trigger is usually done in the probe() routine.

Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210309193620.2176163-4-gwendal@chromium.org


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