ptp: Fix compiler warnings in the testptp utility

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Dong Zhu <bluezhudong@gmail.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>