mt76: enable the VHT extended NSS BW feature

According IEEE Std 802.11-2020, its definition is:
Indicates whether the STA is capable of interpreting the Extended NSS BW
Support subfield of the VHT Capabilities Information field.

Some APs, such as Xiaomi AX6000, would check this one for BW settings.
mt76 driver can get max BW capability only if the this field is confgured
properly.

Reviewed-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Ming Yen Hsieh <mingyen.hsieh@mediatek.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>