It is always appreciable to call people with their original names as we all love to hear our original names with correct pronunciation. You may begin it with one of your friends to make her say your correct name. In this way you will make other people say your name correctly. However, if still some people call you as Usna, they might not be doing this deliberately. There might be some accent problem. Never take it so seriously. I advise you not to bother with such trivial issues. Be patient and learn to face problems for in near future you will have to face and handle lots of complex issues.