qcacld-2.0: extscan: Maintain boot time since extscan start

Firmware is returning the timestamp offset from start of
extscan instead of start from wifi on for extscan cached results.
Thus, change the boot time since wifi on to maintain boot time
since extscan start.
CRs-Fixed: 917695
BUG=24116601
Signed-off-by: Amarnath Hullur Subramanyam <amarnath@codeaurora.org>