avcodec: only warn about hwaccel with frame threads

VLC uses hwaccel with frame threads and it works fine, but returning
an error here made it fail.

This regression was introduced in commit 31741aec.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

Warning message text by nevcairiel
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/utils.c

@@ -1016,9 +1016,8 @@ static int setup_hwaccel(AVCodecContext *avctx,
     int ret        = 0;
 
     if (avctx->active_thread_type & FF_THREAD_FRAME) {
-        av_log(avctx, AV_LOG_ERROR,
-               "Hardware accelerated decoding with frame threading is not supported.\n");
-        return AVERROR(EINVAL);
+        av_log(avctx, AV_LOG_WARNING,
+               "Hardware accelerated decoding with frame threading is known to be unstable and its use is discourage.\n");
     }
 
     if (!hwa) {