tg3: Add phy-related preprocessor constants

This patch replaces some instances of hardcoded phy register values with
preprocessor equivalents.

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>