Bluetooth: Remove the send_lock spinlock from ERTM

Using a lock to deal with the ERTM race condition - interruption with
new data from the hci layer - is wrong. We should use the native skb
backlog queue.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>