ath10k: Enable the MCS8 and MCS9 at 2.4G band

Enable the MCS8 and MCS9 support for 2.4G band, it will
use these data rate with other devices having the same
capability.

Signed-off-by: Yanbo Li <yanbol@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>