net: netcp_ethss: Use the PHY time stamping interface.

The netcp_ethss driver tests fields of the phy_device in order to
determine whether to defer to the PHY's time stamping functionality.
This patch replaces the open coded logic with an invocation of the
proper methods.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>