avcodec/v4l2_context: return EAGAIN to signal full buffers

Return proper error when frame buffers are full. This path is triggered
on the DragonBoard 410c since the encoding API change in commit
827d6fe73d.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>
Reviewed-by: Mark Thompson <sw@jkqxz.net>

libavcodec/v4l2_context.c

@@ -599,7 +599,7 @@ int ff_v4l2_context_enqueue_frame(V4L2Context* ctx, const AVFrame* frame)
 
     avbuf = v4l2_getfree_v4l2buf(ctx);
     if (!avbuf)
-        return AVERROR(ENOMEM);
+        return AVERROR(EAGAIN);
 
     ret = ff_v4l2_buffer_avframe_to_buf(frame, avbuf);
     if (ret)