Fix listing of some configuration options.

Fix listing of some configuration options.

The listing of the following configuration options is currently
broken in sigrok-cli (invalid values are displayed):

 - "timebase" in yokogawa-dlm, rohde-schwarz-hameg, hantek-dso,
   lecroy-xstream, siglent-sds and rigol-ds drivers;
 - "vdiv" in hantek-6xxx, yokogawa-dlm, rohde-schwarz-hameg, hantek-dso,
   lecroy-xstream, hung-chang-dso-2100, siglent-sds and rigol-ds drivers;
 - "spl_meas_range" in pce-322a and cem-dt-885x drivers.

Fixes bug #1331.

Signed-off-by: Guido Trentalancia <redacted>