ath9k: fix length handling of unaggregated frames (might resolve some rts/cts issues)

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

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@38995 3c298f89-4303-0410-b956-a3cf2f4a3e73