altos: Allow setting telemetry interval to zero without crashing

Setting telemetry interval to zero means disable telemetry, but the
code was using the zero value to compute a bunch of packet delays,
which involved dividing by the interval value causing a crash when it
was zero. Skip those subsequent computations as they won't be used
anyways.

Signed-off-by: Keith Packard <keithp@keithp.com>