ath9k: optimize ath9k_flush

Instead of checking the queues in a loop with hardcoded sleep times
inbetween, use a wait queue to trigger queue checks after the tx
processing tasklet has run.

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