ath10k: fix memory leak

In ath10k_usb_hif_tx_sg the allocated urb should be released if
usb_submit_urb fails.

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