net: phy: dp83640: use new PTP_MSGTYPE_SYNC define

Replace use of magic number with recently introduced define.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>