atrf-txrx: new option -r to select data rate
atrf-txrx: new option -r to select data rate

- atrf-txrx.c (set_rate, main): allow setting a non-standard high data
  rate on the AT86RF231
- atrf-txrx.c (usage, main): added option -r to select the data rate

cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan,cfriedt/ben-wpan