iio: humidity: am2315: set up buffer timestamps for non-zero values

Use the iio_pollfunc_store_time parameter during triggered buffer
set-up to get valid timestamps.

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Reviewed-By: Tiberiu Breana <tiberiu.a.breana@intel.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>