ethernet: ti: cpts: Use generic helper function

In order to reduce code duplication between ptp drivers, generic helper
functions were introduced. Use them.

Suggested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>