drm/xe: Change xe_guc_submit_stop return to void

The function xe_guc_submit_stop consistently returns 0 without an error
state, prompting the caller to verify it, which is redundant.

Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240424041911.2184868-1-himal.prasad.ghimiray@intel.com