libcamera: controls: Disallow arrays of arrays

Arrays of arrays, even arrays of strings, are not supported by
the current `ControlValue` mechanism, so disable them for now
to trigger compile time errors if attempts are made to use them.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/controls.h

@@ -120,7 +120,7 @@ struct control_type<Point> {
 };
 
 template<typename T, std::size_t N>
-struct control_type<Span<T, N>> : public control_type<std::remove_cv_t<T>> {
+struct control_type<Span<T, N>, std::enable_if_t<control_type<std::remove_cv_t<T>>::size == 0>> : public control_type<std::remove_cv_t<T>> {
 	static constexpr std::size_t size = N;
 };