altos: Fix ability to disable telemetry by setting interval to 0

altos: Fix ability to disable telemetry by setting interval to 0

For non-zero telemetry intervals, the radio code limits the value
based on the data rate. However, a zero interval means that telemetry
should be entirely disabled, so that value should be left alone when
checking.

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