altosui: Catch attempt to set radio frequency to 0.0 -- use default

altosui: Catch attempt to set radio frequency to 0.0 -- use default

Monitor idle was setting the frequency to 0, which takes a while with
the new native radio frequency setting code. Don't do that, instead
pull out the preferred frequency for that, as is done in other places
where a frequency of 0.0 is used.

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