tg3: Don't access phy test ctrl reg for 5717+

The phy test register location has been repurposed for 5717+ devices.
This patch changes the code to avoid this location for these devices.

Reviewed-by: Benjamin Li <benli@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>