mac80211: don't accept WEP keys other than WEP40 and WEP104

This patch makes mac80211 refuse a WEP key whose length is not WEP40 nor
WEP104.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>