media: siano: Refactor struct sms_msg_data

Replace a single element array with a single element field.

The endianness conversion code seems to support multiple elements. To
avoid changing behavior a pointer to the single element has been used.

This is safer than moving to a flex array, because in that case the
structure size changes.

This fixes the following cocci warning:
drivers/media/common/siano/smscoreapi.h:619:5-13: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays

)

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>