tg3: Add MDIO bus address assignments

The 5717 is a dual port chip that has a shared MDIO bus design.  While
it is impossible for one function to interface with the wrong phy, that
function still needs to know which MDIO bus address to use when
interfacing with its own phy.  This patch adds code to determine which
MDIO bus address to use.

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