ath10k: add max_tx_power for QCA6174 WLAN.RM.2.0 firmware

QCA6174 WLAN.RM.2.0 firmware uses max_tx_power instead of using max_reg_power
to set transmission power. The tx power was about -50dbm, after applying this
change, it become -32dbm.

Signed-off-by: Alan Liu <alanliu@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>