dcb: always initialize priority combo boxes in DCB page

Else the errors like this are shown:
(nm-connection-editor:15976): GLib-GObject-WARNING **: value "-2" of type `gint' is invalid or out of range for property `app-fcoe-priority' of type `gint'

Signed-off-by: Jiří Klimeš <jklimes@redhat.com>