ic->ic_bss, but it uses the rateset in its new ieee80211_node
argument, instead. If the rate is fixed by ic->ic_fixed_rate, but
the fixed rate is not in the node's rateset, choose a reasonable
default: prefer the lowest basic rate or, if there is no basic
rate, prefer the lowest rate, period.
Change a printf complaint to a debug message.
Adapt drivers to suit new ieee80211_get_rate calling convention.
XXX I really need to replace ieee80211_get_rate with a bitrate
XXX adaptation algorithm. Soon, soon....