iio:chemical:sps30: Fix timestamp alignment

One of a class of bugs pointed out by Lars in a recent review.
iio_push_to_buffers_with_timestamp assumes the buffer used is aligned
to the size of the timestamp (8 bytes).  This is not guaranteed in
this driver which uses an array of smaller elements on the stack.

Fixes: 232e0f6d ("iio: chemical: add support for Sensirion SPS30 sensor")
Reported-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: <Stable@vger.kernel.org>
Acked-by: Tomasz Duszynski <tomasz.duszynski@octakon.com>