ath9k: fix possible hang on flush

If a flush is requested, make sure to clear the descriptor once we've
processed it.

This resolves a hang that will occur if all RX descriptors are full when a
flush is requested.

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