drm/xe/guc: Use valid scratch register for posting read

There are only 4 scratch registers VF_SW_FLAG(0..3) on each GuC.
We shouldn't use non-existing register VF_SW_FLAG(4) for posting
read.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>