[Bluetooth] Fix userspace breakage due missing class links

The Bluetooth adapters and connections are best presented via a class
in sysfs. The removal of the links inside the Bluetooth class broke
assumptions by userspace programs on how to find attached adapters.

This patch creates adapters and connections as part of the Bluetooth
class, but it uses different device types to distinguish them. The
userspace programs can now easily navigate in the sysfs device tree.

The unused platform device and bus have been removed to keep the
code simple and clean.

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