Fix MEDIA_LNK_FL_* and MEDIA_PAD_FL_* visibility in CLI translation unit

The CLI is a separate translation unit that doesn't include <linux/media.h>,
so the kernel flag constants were undefined there. Fix by defining our own
public constants in media_ctrl.h:
- MEDIA_LINK_FL_{ENABLED,IMMUTABLE,DYNAMIC} for link flags
- MEDIA_PAD_FLAG_{SINK,SOURCE} for pad flags (distinct names avoid
  redefinition conflict with kernel's MEDIA_PAD_FL_* in media_ctrl.c)

media_ctrl.c now translates kernel flags to our constants on the way out.
Added fallback #defines for kernel constants that were missing them.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

include/media_ctrl.h

@@ -33,10 +33,19 @@ struct Media_Pad {
 	uint32_t flags;      /* MEDIA_PAD_FL_SINK / MEDIA_PAD_FL_SOURCE */
 };
 
+/* Link flags for Media_Link.flags */
+#define MEDIA_LINK_FL_ENABLED    (1u << 0)
+#define MEDIA_LINK_FL_IMMUTABLE  (1u << 1)
+#define MEDIA_LINK_FL_DYNAMIC    (1u << 2)
+
+/* Pad flags for Media_Pad.flags */
+#define MEDIA_PAD_FLAG_SINK    (1u << 0)
+#define MEDIA_PAD_FLAG_SOURCE  (1u << 1)
+
 struct Media_Link {
 	struct Media_Pad source;
 	struct Media_Pad sink;
-	uint32_t         flags; /* MEDIA_LNK_FL_ENABLED, MEDIA_LNK_FL_IMMUTABLE */
+	uint32_t         flags;
 };
 
 /*