altosui: New AltosSerial.set_radio function sets channel/call

Use this anytime you need to set the device radio channel and call
sign, either for telemetry reception or packet mode origination. This
uses the saved callsign and per-device radio channel number. Do not
use this when opening a telemetrum as there won't be a saved channel number.

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