ath10k: report tx airtime provided by fw

If supported, update transmit airtime in mac80211 with the airtime
values reported by the firmware. TX airtime of the PPDU is reported
via HTT data TX completion indication message.

A new service flag 'WMI_SERVICE_REPORT_AIRTIME' is added to advertise
the firmware support. For firmwares which do not support this feature,
TX airtime is calculated in the driver using TX bitrate.

Hardwares tested : QCA9984
Firmwares tested : 10.4-3.6.1-00841

Signed-off-by: Manikanta Pubbisetty <mpubbise@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>