Tx/Rx power values should be rounded up to 3 decimal places (#432)

Signed-off-by: Mihir Patel <patelmi@microsoft.com>

sonic_platform_base/sonic_xcvr/api/xcvr_api.py

@@ -15,7 +15,7 @@ def mw_to_dbm(mW):
             return float("-inf")
         elif mW < 0:
             return float("NaN")
-        return 10. * log10(mW)
+        return round(10. * log10(mW), 3)
 
     def get_model(self):
         """