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.