ath9k: fix another source of corrupt frames

Atheros hardware supports receiving frames that span multiple
descriptors and buffers. In this case, the rx status of every
descriptor except for the last one is invalid and may contain random
data. Because the driver does not support this, it needs to drop such
frames.

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