ptp_message.cpp char is miss used

[liuzuanmin]

Thanks the project firstly.
When try to run it, find one issue.

https://github.com/Avnu/gptp/blob/master/common/ptp_message.cpp#L1964
origin:
char linkDelayInterval = tlv.getLinkDelayInterval();
char timeSyncInterval = tlv.getTimeSyncInterval();
char announceInterval = tlv.getAnnounceInterval();
It would be better if change char to int8_t.