mt76: add phy_idx in mt76_rx_status

Introduce phy_idx mt76_rx_status instead of ext_idx. This is a
preliminary patch to add newer chipset support

Co-developed-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Bo Jiao <bo.jiao@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>