mac80211: clean up eapol handling in TX path

The previous patch left only one user of the ieee80211_is_eapol()
function and that user can be eliminated easily by introducing
a new "frame is EAPOL" flag to handle the frame specially (we
already have this information) instead of doing the (expensive)
ieee80211_is_eapol() all the time.

Also, allow unencrypted frames to be sent when they are injected.

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>