ath9k: always clear ps filter bit on new assoc

commit 026d5b07c03458f9c0ccd19c3850564a5409c325 upstream.

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

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>