ath9k: fix massive rx packet loss issue

This patch fixes a regression introduced in
"ath9k: avoid the copy skb->cb on every RX'd skb"

With that change, the rx status in skb->cb was left uninitialized

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