AP_RCTelemetry: add missing CRSF scheduler table entry

[andyp1per]

Closes #28797

The basic problem was that the VARIO table entry was missing so that when we disabled or changed it we changed a different entry. It also meant the PING entry was never run which is vital to determining RX liveness.

[tpwrules]

Can we check _time_slots directly (maybe through like an assert_task_count(count) in the superclass)? That would also catch if two tasks got assigned the same ID or if we overflowed and ran out of task slots.

On the other hand it might optimize less; the compiler might be able to elide the panic right now.

[peterbarker]

LGTM

[tpwrules]

LGTM, like the smaller change.

But let's not forget to come back and make this less error-prone.