net: dsa: hellcreek: Convert to gettimex64()

As of commit 916444df ("ptp: deprecate gettime64() in favor of
gettimex64()") (new) PTP drivers should rather implement gettimex64().

In addition, this variant provides timestamps from the system clock. The
readings have to be recorded right before and after reading the lowest bits
of the PHC timestamp.

Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>