nl80211: Implement TX of control port frames

This commit implements the TX side of NL80211_CMD_CONTROL_PORT_FRAME.
Userspace provides the raw EAPoL frame using NL80211_ATTR_FRAME.
Userspace should also provide the destination address and the protocol
type to use when sending the frame.  This is used to implement TX of
Pre-authentication frames.  If CONTROL_PORT_ETHERTYPE_NO_ENCRYPT is
specified, then the driver will be asked not to encrypt the outgoing
frame.

A new EXT_FEATURE flag is introduced so that nl80211 code can check
whether a given wiphy has capability to pass EAPoL frames over nl80211.

Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>