chan_simpleusb, chan_usbradio: Reduce frequency of device string not found messages

[KB4MDD]

This changes chan_simpleusb and chan_usbradio to reduce frequency of device string not found messages.

The message will now be repeated every 4.25 minutes.

This closes #229

[tsawyer]

showing the warning just once would be sufficient.

I think that would be best.

[KB4MDD]

Based on the comments, I have revised the code to only print the message one time. If the problem is cleared and occurs again, it will be reported once with the next occurrence.