ptp: Allow selecting trigger/event index in testptp

Currently the trigger/event is hardcoded to 0, this patch adds
a new command line argument -i to select an arbitrary trigger/
event.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>