Bluetooth: Move skb->dev assignment for hdev->send into central place

The assignement of skb->dev is done all over the place. So it makes it
hard to eventually get rid of it. Move it all in one central place so
it gets assigned right before calling hdev->send driver callback.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>