ieee80211: add definition for SMS4 key len

Add SMS4 key length definition to ieee80211_key_len enum.
It's used by WAPI.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>