ath9k: avoid passing buffers to the hardware during flush

The commit "ath9k: fix possible hang on flush" changed the receive code
to always link rx descriptors of processed frames, even when flushing.
In some cases, this leads to flushed rx buffers being passed to the
hardware while rx is already stopped.

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