mac80211: fix off-by-one regression in ibss beacon generation

There is an off-by-one error in the beacon generation for the ibss mode,
falsely a rate the extended supported rates which was already added to
supported rates, messing up the beacon. This was introduced by commit
"mac80211: select and adjust bitrates according to channel mode".

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