ath9k: do not link receive buffers during flush

On AR9300 the rx FIFO needs to be empty during reset to ensure that no
further DMA activity is generated, otherwise it might lead to memory
corruption issues.

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