ath9k_htc: Fix memory leak

commit 0981c3b24ef664f5611008a6e6d0622fac6d892b upstream.

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.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>