net80211: fix out-of-bounds read in ieee80211_amrr(9).

ieee80211_alloc_node() does not initialize rateset tables; that's not
expected by rate control modules and will result in array access at
index -1 - where ni_essid[] array is located (zeroed at allocation, so
there are no user-visible consequences).

Just delay rate control initialization to the moment, when rateset
tables are initiaziled; nothing will use rates here anyway.

MFC after: 4 days