ath9k: fix bogus sequence number increases on aggregation tid flush

When a tid pointer is passed to ath_tx_send_normal(), it increases the
starting sequence number for the next AMPDU action frame, which should
only be done if the sequence number assignment is fresh. In this case
it is clearly not.

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