iio: accel: stk8312: Fix buffer alignment in iio_push_to_buffers_with_timestamp()

iio: accel: stk8312: Fix buffer alignment in iio_push_to_buffers_with_timestamp()

To make code more readable, use a structure to express the channel
layout and ensure the timestamp is 8 byte aligned.

Found during an audit of all calls of this function.

Fixes: 95c12bba51c3 ("iio: accel: Add buffer mode for Sensortek STK8312")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20210501170121.512209-7-jic23@kernel.org