ath6kl: Fix potential memory leak in ath6kl_tx_complete()

We bail out from ath6kl_tx_complete() if any of the sanity
checks on skb and ath6kl_cookie fails. By doing this we
potentially leak few remaining buffers in packet_queue.
Make sure to proceed processing the remaining buffers
as well. This issue is found during code review.

Reported-by: Wang yufeng <yufengw@qca.qualcomm.com>
Signed-off-by: Vasanthakumar Thiagarajan <vthiagar@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>