cfg80211/nl80211: Add support for NL80211_STA_INFO_RX_DURATION

Add support for the a station statistics netlink attribute:
NL80211_STA_INFO_RX_DURATION.

If present, this attribute contains the aggregate PPDU duration (in
microseconds) for all the frames from the peer. This is useful to
help understand the total time spent transmitting to us by all of
the connected peers.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>