Bluetooth: Add support hdev to allocate private data

This patch adds support hdev to allocate extra size for private data.
The size of private data is specified in the hdev_alloc_size(priv_size)
and the allocated buffer can be accessed with hci_get_priv(hdev).

Signed-off-by: Tedd Ho-Jeong An <tedd.an@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>