wifi: mac80211: fix double SW scan stop

When we stop a not-yet-started scan, we erroneously call
into the driver, causing a sequence of sw_scan_start()
followed by sw_scan_complete() twice. This will cause a
warning in hwsim with next in line commit that validates
the address passed to wmediumd/virtio. Fix this by doing
the calls only if we were actually scanning.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>