ath6kl: implement rx flush for htc pipe

rx flush was not implemented for htc pipe, add that now. Doesn't fix any known
issues.

Also free the skb if htc control messages get canceled.

Signed-off-by: Jessica Wu <wjessica@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>