Bluetooth: L2CAP: Add l2cap_le_flowctl_send

Consolidate code sending data to LE CoC channels and adds proper
accounting of packets sent, the remaining credits and how many packets
are queued.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>