Fix sub-underfow when converting scan result's age from ms to us

Commit 5a441fcf ('Fix mul-overflow when converting scan result's
age from ms to us') fixes mul-overflow, but still sub-underflow
can happen in same line due to data type is u64, fix this by
moving check to earlier lines to ensure the sub does not results
in a negative value.

Bug: 387187200
Test: m
Change-Id: I81035e749d627fa3a7901b672241488963f3e046