ieee80211_start_roc_work() might add a new roc
to existing roc, and tell cfg80211 it has already
started.

However, this might happen before the roc cookie
was set, resulting in REMAIN_ON_CHANNEL (started)
event with null cookie. Consequently, it can make
wpa_supplicant go out of sync.

Fix it by setting the roc cookie earlier.

Cc: stable@vger.kernel.org
Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>