ath9k: Do not indicate RX_FLAG_DECRYPTED for unprotected frames

mac80211 skips drop_unencrypted checks if the driver/firmware has
already taken care of this. In case of ath9k, we must not indicate
that the frame was decrypted if no decryption was actually done.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>