iio:bmi160: use iio_pollfunc_store_time

Currently, we snap the timestamp after reading from the buffer and
processing the event. When the IIO poll function is triggered by an
interrupt,  we can get a slightly more accurate timestamp by snapping it
prior to reading the data, since the data was already generated prior to
entering the trigger handler. This is not going to make a huge difference,
but we might as well improve slightly.

Do this by using iio_pollfunc_store_time as other drivers do.

Signed-off-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>