Bluetooth: Correctly take hci_dev->dev refcount

The hci_dev->dev device structure has an internal refcount. This
refcount is used to protect the whole hci_dev object. However, we
currently do not use it.  Therefore, if someone calls hci_free_dev() we
currently immediately destroy the hci_dev object because we never took
the device refcount.

This even happens if the hci_dev->refcnt is not 0. In fact, the
hci_dev->refcnt is totally useless in its current state. Therefore, we
simply remove hci_dev->refcnt and instead use hci_dev->dev refcnt.

This fixes all the symptoms and also correctly integrates the device
structure into our bluetooth bus system.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>