ath9k: fix the .flush driver op implementation

This patch simplifies the flush op and reuses ath_drain_all_txq for
flushing out pending frames if necessary. It also uses a global timeout
of 200ms instead of the per-queue 60ms timeout.

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