ptp: dp83640: convert to the 64 bit get/set time methods.

This device stores the number of seconds in a 32 bit register, and the
stored value is unsigned.  Therefore this driver and device are ready
for the year 2038.  However, more work will be needed prior to 2106.

Compile tested only.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>