ath9k_htc: Fix memory leak

SKBs that are allocated in the HTC layer do not have callbacks
registered and hence ended up not being freed, Fix this by freeing
them properly in the TX completion routine.

Cc: <stable@vger.kernel.org>
Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>