ath10k: fill tx_duration for each peer in Tx stats per STA

Firmware sends the tx_duration for each in HTT_T2H_MSG_TYPE_PEER_STATS
msg. Fill the tx_duration sent by firmware in the tx stats information
per STA.

Tested HW: WCN3990
Tested FW: WLAN.HL.3.1-00784-QCAHLSWMTPLZ-1,
           WLAN.HL.2.0-01617-QCAHLSWMTPLZ-1

Signed-off-by: Surabhi Vishnoi <svishnoi@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>