mac80211: fix tx-mgmt cookie value being left uninitialized

commit "mac80211: unify SW/offload remain-on-channel"
moved the cookie assignment from ieee80211_mgmt_tx()
to ieee80211_start_roc_work().  But the latter is only
called where offchannel is needed.  If offchannel isn't
needed/used, a uninitialized cookie value would be returned
to userspace.

This patch sets the cookie value when offchannel isn't used.

Signed-off-by: Nicolas Cavallari <cavallar@lri.fr>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>