iio: adc: ltc2309: Optimize chip_info structure layout

iio: adc: ltc2309: Optimize chip_info structure layout

Improve the ltc2309_chip_info structure with better type safety and
memory efficiency:

- Add __counted_by_ptr() annotation to the channels pointer, linking
  it to num_channels for improved bounds checking and kernel hardening
- Reorder structure fields to minimize padding:
  * Place read_delay_us before num_channels
  * This reduces struct size and eliminates internal gaps
- Reorder field initialization to match the structure definition order

The __counted_by_ptr() attribute enables compile-time and runtime
verification that array accesses to channels[] stay within the bounds
specified by num_channels, improving memory safety.

Signed-off-by: Carlos Jones Jr <carlosjr.jones@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Reviewed-by: Andy Shevchenko <andy@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>