Bluetooth: Make mgmt_set_class_of_dev_complete() return void

The return value of mgmt_set_class_of_dev_complete() function is
not used and so just change it to return void.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>