kernel/802_11: More ratectl fixes (fixes an ath(4) panic).

Set the default rate control algorithm to NONE instead of AMRR and
add some missing ieee80211_ratectl_node_init()s which I had overlooked
earlier.

Reported-by: Johannes Hofmann <johannes.hofmann@gmx.de>
Tested-by:   Imre Vadasz <imre@vdsz.com>