When CONFIG_DMI is not enabled, dmi detection should flag that no board
could be detected (err=1) rather than another error condition (err<0).

This fixes the fallback to manual probing for all motherboards, even
those without DMI strings, when CONFIG_DMI=n.
Signed-off-by: Alistair John Strachan <alistair@devzero.co.uk>
Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>