nl80211: Add CMD_CONTROL_PORT_FRAME API

This commit also adds cfg80211_rx_control_port function.  This is used
to generate a CMD_CONTROL_PORT_FRAME event out to userspace.  The
conn_owner_nlportid is used as the unicast destination.  This means that
userspace must specify NL80211_ATTR_SOCKET_OWNER flag if control port
over nl80211 routing is requested in NL80211_CMD_CONNECT,
NL80211_CMD_ASSOCIATE, NL80211_CMD_START_AP or IBSS/mesh join.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
[johannes: fix return value of cfg80211_rx_control_port()]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>