ath9k: fix a regression in PS frame filter handling

Only leave filtering enabled for AP or VLAN interfaces, clear the
destination mask for all other interfaces.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Reported-by: Ben Greear <greearb@candelatech.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>