wifi: ath11k: add get_txpower mac ops

Driver does not support get_txpower mac ops because of which
cfg80211 returns vif->bss_conf.txpower to user space. bss_conf.txpower
gets its value from ieee80211_channel->max_reg_power. However, the final
txpower is dependent on few other parameters apart from max regulatory
supported power. It is the firmware which knows about all these
parameters and considers the minimum for each packet transmission.

All ath11k firmware reports the final tx power in firmware pdev stats
which falls under fw_stats.

Add get_txpower mac ops to get the tx power from firmware leveraging
fw_stats and return it accordingly.

Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.5.0.1-01100-QCAHKSWPL_SILICONZ-1
Tested-on: QCN9074 hw1.0 PCI WLAN.HK.2.5.0.1-01100-QCAHKSWPL_SILICONZ-1
Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3

Signed-off-by: Aditya Kumar Singh <quic_adisi@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220603082814.31466-3-quic_adisi@quicinc.com