ath10k: use VHT phymodes for 20/40MHz channels

chandef does not provide 11n / 11ac split. Use the highest
phymodes for 20/40MHz channels, firmware can operate on.

Signed-off-by: Bartosz Markowski <bartosz.markowski@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>