cfg80211: fix rcu in cfg80211_unregister_wdev

Callers of cfg80211_unregister_wdev can free the wdev object
immediately after this function returns. This may crash the kernel
because this wdev object is still in use by other threads.
Add synchronize_rcu() after list_del_rcu to make sure wdev object can
be safely freed.

Change-Id: Id73921888bed032312b133e94fbfe06537e06b92
Signed-off-by: Dedy Lansky <dlansky@codeaurora.org>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Git-commit: bf2b61a6838f19cbc33f6732715012c483fa3795
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git