shell: backends: rtt: Set panic_mode flag

A bug was introduced when fb17d0e365 introduced host presence detection.
Panic_mode flag was added which indicates that RTT should work in
synchronous, blocking mode but it was not set when backend was switched
to panic mode.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>