mac80211: return correct error return from ieee80211_wep_init

Return the proper error code rather than a hard-coded ENOMEM from
ieee80211_wep_init.  Also, print the error code on failure.

Signed-off-by: Jeremy Fitzhardinge <jeremy@goop.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>