wifi: mt76: mt7996: enable VHT extended NSS BW feature

Set SUPPORTS_VHT_EXT_NSS_BW to let the max BW capability correctly be
parsed by different devices.

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