[fix] Fixed FreeRADIUS PostAuthView returning 500 error openwisp#467

Set the max_length attribute to be equal to 50 on the called_station_id
field, so that it doesn't give a 500 error(400 error instead) every time
it exceeds 50.

Fixes openwisp#467

openwisp_radius/api/serializers.py

@@ -120,7 +120,7 @@ class RadiusPostAuthSerializer(serializers.ModelSerializer):
         allow_blank=True,
         style={'input_type': 'password'},
     )
-    called_station_id = serializers.CharField(required=False, allow_blank=True)
+    called_station_id = serializers.CharField(required=False, allow_blank=True, max_length=50)
     calling_station_id = serializers.CharField(required=False, allow_blank=True)
 
     def validate(self, data):