rc80211_pid should respect fixed rates.

I would argue that mac80211 should handle fixed rates outside the rate
control code, which would also allow them to take effect immediately
instead of during the rate control callback, but this is pretty close
to correct.

Signed-Off-By: Andy Lutomirski <luto@myrealbox.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>