mac80211: refactor the WEP code to be directly usable

The new key work for cfg80211 will only give us the WEP
key for shared auth to do that authentication, and not
via the regular key settings, so we need to be able to
encrypt a single frame in software, and that without a
key struct. Thus, refactor the WEP code to not require
a key structure but use the key, len and idx directly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>