iio: adc: ti-ads8688: fix timestamp is not updated in buffer

When using the hrtimer iio trigger timestamp isn't updated.
If we use iio_get_time_ns it is updated correctly.

Fixes: 2a864877 ("iio: adc: ti-ads8688: add trigger and buffer support")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>