ath9k: reset buffer stale flag in ath_tx_get_tid_subframe

If that flag stays set for a buffer that already ran through the tx path
once, it might cause issues in tx completion processing. Better clear it
early to ensure that this does not happen

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