ENGCOM-1433: [Backport] Fix faulty admin spinner animation #14940

 - Merge Pull Request #14940 from rogyar/magento2:2.1-spinner-animation-fix
 - Merged commits:
   1. 91857b0

Author: magento-engcom-team

app/design/adminhtml/Magento/backend/web/css/source/variables/_spinner.less

@@ -26,4 +26,4 @@
 @spinner-rotate-step: 45;
 
 //  One step in degree
-@spinner-delay: .9;
+@spinner-delay: .9s;