cfg80211: fix ibss wext chandef creation

The wext internal chandefs for ibss should be created using the
cfg80211_chandef_create() functions. Initializing fields manually is
error-prone.

Reported-by: Dirk Gouders <dirk@gouders.net>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>