ath5k: 5211, don't crypt every protected frame

Set null key type even on ar5211, otherwise it en/decrypts every frame with
protected bit set which renders the card unusable on encrypted networks.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>