mac80211: avoid reconfig if no interfaces are up

If there are no interfaces up, there is no reason
to continue the reconfig flow.

The current code might end up calling driver
callbacks (e.g. resume(), reconfig_complete())
while the driver is already stopped.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>