ath9k: fix ar956x device detection

Signed-off-by: Felix Fietkau <nbd@openwrt.org>

Backport of r46090

git-svn-id: svn://svn.openwrt.org/openwrt/branches/chaos_calmer@46091 3c298f89-4303-0410-b956-a3cf2f4a3e73