mac80211: avoid NULL ptr deref when using WEP

"mac80211: move control.hw_key assignment" changed an if-else into two
separate if statments, but the if-else is needed to prevent
dereferencing a null info->control.hw_key.  This fixes avoids a lock-up
during association on my machine when using WEP.

Signed-off-by: John W. Linville <linville@tuxdriver.com>