ath9k: release allocated buffer if timed out

In ath9k_wmi_cmd, the allocated network buffer needs to be released
if timeout happens. Otherwise memory will be leaked.

Signed-off-by: Navid Emamdoost <navid.emamdoost@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
