tools/dirtpan/dirtpan.c (rx_pck): break tie if both sides are in s_tx
tools/dirtpan/dirtpan.c (rx_pck): break tie if both sides are in s_tx

Note that we still waste at least one perfectly good frame. However,
instead of implementing some fancy arbitration, it's better if we
just make the whole thing bidirectional in the future.

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