cfg80211: Disallow HT/WEP in IBSS mode

Currently, a user is allowed to choose a HT operating channel
with WEP when creating an IBSS network. WEP is not allowed
in HT configuration - this patch ensures that such requests
are denied.

Signed-off-by: Sujith Manoharan <c_manoha@qti.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>