[Nokia ixs7215] Platform API 2.0 improvements

Fix bug detected by LGTM
  - multiply defined function name

Author: dflynn-Nokia

platform/marvell-armhf/sonic-platform-nokia/7215/sonic_platform/chassis.py

@@ -159,11 +159,11 @@ def get_model(self):
         """
         return self._eeprom.part_number_str()
 
-    def get_serial(self):
+    def get_service_tag(self):
         """
-        Retrieves the serial number of the chassis (Service tag)
+        Retrieves the Service Tag of the chassis
         Returns:
-            string: Serial number of chassis
+            string: Service Tag of chassis
         """
         return self._eeprom.service_tag_str()