tg3: Delay mdio bus init until fw finishes

The device firmware uses the MDIO bus during early setup.  If the driver
modifies the MDIO bus configuration while it is in use by the firmware,
any number of bad things can happen.  This patch delays MDIO setup until
after the firmware posts its magic signature, signifying initialization
is complete.

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