net:phy:dp83640: Do not hardcode timestamping event edge

net:phy:dp83640: Do not hardcode timestamping event edge

Currently the external timestamping code is hardcoded to use the
rising edge even though the hardware has configurable event edge
detection. This patch changes the code to use falling edge detection
if PTP_FALLING_EDGE is set in the user supplied flags.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>