[Bluetooth] Attach host adapters to the Bluetooth bus

The Bluetooth host adapters are attached to the Bluetooth class and the
low-level connections are children of these class devices. Having class
devices as parent of bus devices breaks a lot of reasonable assumptions
about sysfs. The host adapters should be attached to the Bluetooth bus
to simplify the dependency resolving. For compatibility an additional
symlink from the Bluetooth class will be used.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>