wifi: libertas: fix WARNING in usb_tx_block

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.

Reported-by: syzbot+67969ab6a2551c27f71b@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=67969ab6a2551c27f71b
Signed-off-by: Szymon Wilczek <swilczek.lx@gmail.com>
Link: https://patch.msgid.link/20251221155806.23925-1-swilczek.lx@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>