cfg80211: Add an attribute to set inactivity timeout in AP mode

This patch adds an attribute, NL80211_ATTR_INACTIVITY_TIMEOUT,
to set the inactivity timeout which can be used to remove the
station in AP mode. This can be passed in NL80211_CMD_START_AP
and used by the drivers which have AP MLME in firmware but
don't support get_station() properly. To disable inactivity
timer in userspace, wpa_s for example, there is a new flag,
NL80211_FEATURE_INACTIVITY_TIMER, in nl80211_feature_flags
through which drivers can register their capability to use
the inactivity timeout to free the stations.

Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>