scpi: Raise severity when IDN response lacks the serial number field.

scpi: Raise severity when IDN response lacks the serial number field.

Raise the diagnostics message's severity from debug to warn when the
'*IDN?' response lacks the serial number field. Although it has only
been seen for some GWInstek DMMs, it violates the SCPI spec, and more
or other activity is required in a future implementation. This change
amends commit 47e7a6395e21.