cfg80211/nl80211: implement station attribute retrieval

After a station is added to the kernel's structures, userspace
has to be able to retrieve statistics about that station, especially
whether the station was idle and how much bytes were transferred
to and from it. This adds the necessary code to nl80211.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>