Bluetooth: SCO: Fix sco_send_frame returning skb->len

The skb in modified by hci_send_sco which pushes SCO headers thus
changing skb->len causing sco_sock_sendmsg to fail.

Fixes: 0771cbb3 ("Bluetooth: SCO: Replace use of memcpy_from_msg with bt_skb_sendmsg")
Tested-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>