mac80211: allow driver to iterate keys

When in suspend/wowlan, devices might implement crypto
offload differently (more features), and might require
reprogramming keys for the WoWLAN (as it is the case
for Intel devices that use another uCode image). Thus
allow the driver to iterate all keys in this context.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>