net: phy: dp83640: Use generic helper function

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

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>