wifi: mt76: connac: fix EHT phy mode check

Add a BSS eht_support check before returning EHT phy mode. Without this
patch, there might be an inconsistency where the softmac layer thinks
the BSS is in HE mode, while the FW thinks it is in EHT mode.

Signed-off-by: MeiChia Chiu <meichia.chiu@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>