Bluetooth: Introduce to_hci_dev()

We currently use dev_set_drvdata to keep a pointer to ourself. This
doesn't make sense as we are the bus and not a driver. Therefore,
introduce to_hci_dev() so we can get a struct hci_dev pointer from a
struct device pointer.

dev_set/get_drvdata() is reserved for drivers that provide a device and
not for the bus using the device. The bus can use simple pointer
arithmetic to retrieve its private data.

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>