tg3: Refine phylib support

This patch refines the phylib support in the tg3 driver.  The patch does
the following things :

* Rename tg3_mdio_config() to tg3_mdio_config_5785().  The 5785 will be
  the only device that will use it so the name might as well reflect
  that.
* Fix a memory leak if mdiobus_register() fails.
* Add code to deal with phy device detection failures.
* Add code to correct the supported list of phy features based on the
  MAC <=> PHY interface.

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