Northbound doesn't allow priority and multicastPriority values to be set to 255

When trying to register a mapping using the northbound API, a values above 127 cause the following error: "Internal Error : There was an error while converting the map register".

The yang model defines these fields as uint8, but they seem to be used as signed in the NB API. When a map-register packet with one of those fields set to 255 is sent, it is processed correctly by the southbound API, however, the northbound shows it as a signed integer: -1. Still input of signed integers is not accepted (setting priority to -1 gives the same error).