mt76: add phy_idx to mt76_wcid

Introduce phy_idx to mt76_wcid structure instead of ext_phy.
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>