[T2][Anchor] Fix type check in prefix_list script (#23803)

Fix incorrect checking introduced by #22337
When invoking prefix_list script in supported roles, the checking function should return rather than exit 0. Exiting 0 would directly terminate the script

Author: yaqiangz

dockers/docker-fpm-frr/base_image_files/prefix_list

@@ -39,7 +39,7 @@ check_spine_router() {
 
     # only supported on UpstreamLC or UpperSpineRouter
     if [[ ("$type" == "SpineRouter" && "$subtype" == "UpstreamLC") || "$type" == "UpperSpineRouter" ]]; then
-        exit 0
+        return
     fi
 
     echo "Operation is only supported on Upstream SpineRouter." >&2