USB fast connect/disconnect crash fix

When USB plugged/unplugged at fast rate,
lan78xx_mdio_init() in lan78xx_bind() failing case is not handled.
Whenever  lan78xx_mdio_init() failed, dev->mdiobus will be freed, however
since lan78xx_bind() not consider as error and try to proceed for
further initialization in lan78xx_probe() which leads system hung/crash.
Also when register_netdev() failed, netdev is freed without calling lan78xx_unbind().
Hence halting the failed cases right manner fixes the system crash/hung issue.
Signed-off-by: Nisar Sayed <Nisar.Sayed@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>