cfg80211: validate key index better

Don't accept it if a key_idx < 0 snuck through, reject WEP keys with
key index 4 and 5 (which are used for IGTKs) and don't allow IGTKs
with key indices other than 4 and 5. This makes the key data match
expectations better.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>