mac80211: fix regression when initializing ibss wmm params

There appear to be two regressions in ibss.c when calling
ieee80211_sta_def_wmm_params():
 * the second argument should be a rate length, not a rate array. This
   was introduced by my commit "mac80211: select and adjust bitrates
   according to channel mode"
 * the third argument is not initialized (anymore), making further
   checks within this function useless.

Since ieee80211_sta_def_wmm_params() is only used by ibss anyway,
remove the function entirely and handle the operating mode decision
immediately.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>