wifi: libertas: fix WARNING in usb_tx_block

In the Linux kernel, the following vulnerability has been resolved: wifi: libertas: fix WARNING in usb_tx_block The function usb_tx_block() submits cardp->tx_urb without ensuring that any previous transmission on this URB has completed. If a second call occurs while the URB is still active (e.g. during rapid firmware loading), usb_submit_urb() detects the active state and triggers a warning: 'URB submitted while active'. Fix this by enforcing serialization: call usb_kill_urb() before submitting the new request. This ensures the URB is idle and safe to reuse.