Bluetooth: btwilink: Fix unexpected skb free

The caller (hci_core) still owns the skb in case of error, releasing
it inside the send function can lead to use-after-free errors.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Loic Poulain <loic.poulain@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>