Check ndesc of q_rx to avoid potential hole in iteration. This is an intermediate patch to add mt7986 support. Signed-off-by: Sujuan Chen <sujuan.chen@mediatek.com> Signed-off-by: Bo Jiao <Bo.Jiao@mediatek.com> Reviewed-by: Ryder Lee <ryder.lee@mediatek.com> Signed-off-by: Felix Fietkau <nbd@nbd.name>