Fix issue with inconsistent formatting of "GPS off" buses.

OC Transpo usually reports a bus with no GPS data as having a
"GPS last read" time of (-1) minutes ago.  But sometimes it reports
it as (-2), for reasons that aren't clear (at least, not to me).
The code was applying an "== (-1)" test in one spot, but a "< 0"
test in another; inconsistency is not good, nor is repeating a
conditional test unnecessarily.