router advertisements: fix dhcp6 start on 'unamaged'
router advertisements: fix dhcp6 start on 'unamaged'

We set the mode for radvd config, not for preventing users to
configure a DHCP6 while in unmanaged mode.

PR: https://github.com/opnsense/core/issues/1258

opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core,opnsense/core