Bluetooth: Remove redundant hdev->parent field

We initialize the "struct device" in hci_alloc_dev() for a long time now
so we can access hdev->dev.parent directly. Hence, we can drop the
temporary field hdev->parent which is used in no other place than
hci_add_sysfs().

SET_HCIDEV_DEV() is never called after registering a device by the
drivers so we do not overwrite internal device-state. Furthermore,
hdev->dev is initialized to 0 by kzalloc() inside hci_alloc_dev() so the
default behavior with dev.parent = NULL is kept.

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>