greybus: timesync: Initialize the timesync ping fields to zero

Remember to initialize the TimeSync ping fields to zero so that if a
timesync_get_last_event() returns an error - we display a FrameTime that is
obviously incorrect.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>