Bluetooth device hardware addresses won't change during the lifetime
of the object (since that would mean a completely new device) and
they also won't have an ifindex because they aren't netdevices.

Various bits of the core periodically call nm_device_update_hw_address()
to update a device's hardware address, but this function expects that
any device with a hardware address also has an ifindex.  Except that
Bluetooth devices don't because they aren't netdevices.

Modify the get_hw_address_length() function to return a boolean
indicating whether or not the address can ever change, and set that
for BT devices.  nm_device_update_hw_address() then exits early if
there's no point in re-checking the hardware address, avoiding the
assertion.

https://bugzilla.gnome.org/show_bug.cgi?id=701744