ethtool: mm: fix get_mm() return code not propagating to user space

If ops->get_mm() returns a non-zero error code, we goto out_complete,
but there, we return 0. Fix that to propagate the "ret" variable to the
caller. If ops->get_mm() succeeds, it will always return 0.

Fixes: 2b30f829 ("net: ethtool: add support for MAC Merge layer")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230206094932.446379-1-vladimir.oltean@nxp.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/ethtool/mm.c

@@ -56,7 +56,7 @@ static int mm_prepare_data(const struct ethnl_req_info *req_base,
 out_complete:
 	ethnl_ops_complete(dev);
 
-	return 0;
+	return ret;
 }
 
 static int mm_reply_size(const struct ethnl_req_info *req_base,