Currently instant rx_duration always fetching as zero
in fw_stats debugfs entry if extended peer stats event
supports.

This patch updates instant rx_duration in fw_stats entry
based on extended peer stats and maintaining backward
compatibility for 10.2/10.x.

Tested HW: QCA9984.
Tested FW: 10.4-3.6.0.1-00004.
Signed-off-by: Balaji Pothunoori <bpothuno@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>