examples/default: fix for 802.15.4 radio devices

This adapts the shell command `txtsnd` for 802.15.4 capable radio devices.

**known issues**:
* source address only gets displayed correctly when payload is >=8 characters (?!?)
* payload length on receiver side is zero
Same as https://github.com/RIOT-OS/RIOT/pull/1674/files#r18089039