nl80211 driver: correctly set the encrypt bit for eapol frames

nl80211 driver: correctly set the encrypt bit for eapol frames

A recent kernel change led to all EAPOL frames being encrypted rather than
just those for the group handshake. This is due to transmit processing in
the kernel now using the proper interface which would encrypt those frames
with the group key because hostapd wasn't requesting that they not be
encrypted. This changes the nl80211 driver to not request encryption unless
the EAPOL frame should be encrypted.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>