mt76: redefine mt76_for_each_q_rx to adapt mt7986 changes

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>