tools/arch/x86/intel_sdsi: Simplify ascii printing

Add #define for feature length and move NUL assignment from callers to
get_feature().

Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240411025856.2782476-9-david.e.box@linux.intel.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>