ath9k: always clear ps filter bit on new assoc

Otherwise in some cases, EAPOL frames might be filtered during the
initial handshake, causing delays and assoc failures.

Cc: stable@vger.kernel.org
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>