ath9k: only drop packets in drv_flush when asked to

ath9k: only drop packets in drv_flush when asked to

Recently more places in mac80211 call drv_flush to ensure proper order
for state changes wrt. powersave, channel changes, etc. On some systems
such calls lead to spurious logspam about failing to stop tx dma, as well
as hardware resets that go along with that.
Instead of dropping packets in a place where it's completely unnecessary,
only do it when drop == true.

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