mac80211: clean up eapol frame handling/port control

This cleans up the eapol frame handling and some related code in the
receive and transmit paths. After this patch
 * EAPOL frames addressed to us or the EAPOL group address are
   always accepted regardless of whether they are encrypted or not
 * other frames from a station are dropped if PAE is enabled and
   the station is not authorized
 * unencrypted frames (except the EAPOL frames above) are dropped if
   drop_unencrypted is enabled
 * some superfluous code that eth_type_trans handles anyway is gone
 * port control is done for transmitted packets

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>