AP_RCTelemetry: throttle CRSF request RX device info messages

[olliw42]

I'm not sure you'll like it, but I like it a lot and hence dare to suggest:

There are situations in which one wants to use CRSF, but only with one wire (receiver Tx -> ardupilot Rx), without the additional telemetry wire (receiver Rx <- ardupilot Tx). Examples would be companions or third-party receivers. In such cases the RCTelemetry library sends 50 statustest messages at 2 Hz rate, i.e., for a total of 25 seconds, before it concludes it can stop requesting a CRSF message. The PR here throttles the sending of these statustext messages to one every 5 seconds, which should be really sufficient to have the full attention of the user.

Alternatives: One could add an option which either supresses sending these messages (similar to RC_Channels::Option::SUPPRESS_CRSF_MESSAGE) or which disables the requesting. I think the simple throttling is preferable since it doesn't need user interactions nor any documentation.

The code change has been massively tested for a long while on a copter.

Behavior before this PR:

Behavior after this PR:

[andyp1per]

This can also happen even if you have two wires connected with ELRS.