fix "Time Riding" for rides without kph

"Time Riding" was only calculated when the ride had kph values - although
it's then also including time without kph but cadence. This caused it to
skip all time spent on the trainer (... if there's no sensor on the rear
wheel).

This patch makes it consistently calculate "Time Riding" if there's kph or
cad... might be worth to count time with pwr or similar running metrics,
too.
Merge pull request #1109 from rclasen/timeriding

fix "Time Riding" for rides without kph