Bluetooth: L2CAP: Make `bt_l2cap_send_pdu()`

This API replaces `bt_l2cap_send()` and `bt_l2cap_send_cb()`.

The difference is that it takes the `struct bt_l2cap_le_chan` object
directly instead of a connection + CID.

We need the channel object in order to put the PDU on the TX queue. It
is inefficient to do a search for every PDU when the caller knows the
channel object's address and can just pass it down.

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>