iio: adis16400: Set timestamp as the last element in chan_spec

This is necessary since timestamp is calculated as the last element
in iio_compute_scan_bytes().

Without this fix any userspace code reading the layout of the buffer via
sysfs will incorrectly interpret the data leading some nasty corruption.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>