mt76: connac: fix a theoretical NULL pointer dereference in mt76_connac_get_phy_mode

Even if it is not a real bug since mt76_connac_get_phy_mode runs just
for mt7921 where only STA is supported, fix a theoretical NULL pointer
dereference if new added modes do not support HE

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>