cfg80211: stop sched scan only when needed

cfg80211_leave stops sched scan when any station vif
is leaving. Add an explicit check and call it only
when the relevant vif (the one we scan on) is leaving.

Signed-off-by: Barak Bercovitz <barak@wizery.com>
[Eliad - changed the commit message a bit]
Signed-off-by: Eliad Peller <eliad@wizery.com>
[Johannes - add ASSERT_RTNL since that protects the pointer]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>