tg3: Convert PHY_ADDR => TG3_PHY_MII_ADDR

This patch renames the PHY_ADDR preprocessor definition.  The following
patch will identify a new member on the MDIO bus, so we want this
preprocessor definition to be a little more descriptive.

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