Bluetooth: Remove hci_dev->driver_data

The linux device model provides dev_set/get_drvdata so we can use this
to save private driver data.
This also removes several unnecessary casts.

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>