media: pci: cx18: Use flex arrays for struct cx18_scb

Replace the old-style single element array with a flexible array.
This structure does not seem to be allocated in the code, so there is no
need to change anything else.

The following cocci warning is fixed:
drivers/media/pci/cx18/cx18-scb.h:261:22-29: 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>