cfg80211: set_default_key only for WEP

We invoke the cfg80211 set_default_key callback only for WEP key
configuring.

Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>