This prevents the mavlink transmit loop from waiting on the module mutex
thus not stopping transmissions when the mutex is already taken.
This can happen when calling `mavlink status` from the mavlink shell,
where `Mavlink::get_status_all_instances()` takes the mutex and then
prints the status via pipes to the mavlink transmit buffer.
If that pipe cannot be emptied a deadlock happens.
Since the MavlinkReceiver thread also waits on the module mutex, both
reception and transmission of Mavlink packets are then prevented thus
disabling communications entirely.
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