Fix Tx rate used by rtwn(4) and urtwn(4) for RTS frames.

Fix Tx rate used by rtwn(4) and urtwn(4) for RTS frames.

Using ni_txrate for RTS is a bad choice since it could go up to 54 Mbit/s.
The AP needs to receive our RTS frame reliably. Usually, 1 Mbit/s is used
for RTS, but this hurts throughput and does not really make sense on today's
wifi networks.

Use 1 Mbit/s in 11b mode, and otherwise use 24 Mbit/s, as damien@
already hard-coded in urtwn long ago.

ok kevlo@