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>