PdoMap.save() does not handle cases where 1A00h through 1A03h are read-only

[erlend-aasland]

I've got a sensor device where the TPDO mapping parameters are read-only. AFAICS from the CiA 301 standard, TPDO mappings can be read-only, so it seems to me this case should be supported. A fix should be trivial; I'll propose a patch shortly.