ath9k: fix PS-Poll reception on AR9160 and earlier

I can't find any valid reason for not setting the ATH9K_RX_FILTER_PSPOLL
flag on older hardware and neither the documentation nor the reference
code mention any reason for excluding older hardware here.

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