mt76: do not check the ccmp pn for ONLY_MONITOR frame

if the received frame enables RX_FLAG_ONLY_MONITOR,
driver doesn't need to check the ccmp pn of this frame.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: MeiChia Chiu <MeiChia.Chiu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>