ath9k: Fix potential use-after-free.

The ath_debug_stat_tx references bf->bf_mpdu, which
is the skb consumed by 	ath_tx_complete.  So, call
the ath_debug_stat_tx method first.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>