ath9k: do not call ath9k_hw_txprocdesc on AR9003 outside of the tx tasklet

Since AR9003 uses a global tx status queue, processing tx status outside of
the regular tx tasklet is dangerous and messes up hardware/software
synchronization of tx status events.

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