cfg80211: make WoWLAN configuration available to drivers

Make the current WoWLAN configuration available to drivers
at runtime. This isn't really useful for the normal WoWLAN
behaviour and accessing it can also be racy, but drivers
may use it for testing the WoWLAN device behaviour while
the host stays up & running to observe the device.

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