net: Do not fire linkwatch events until the device is registered.

Several device drivers try to do things like netif_carrier_off()
before register_netdev() is invoked.  This is bogus, but too many
drivers do this to fix them all up in one go.

Reported-by: Folkert van Heusden <folkert@vanheusden.com>
Signed-off-by: David S. Miller <davem@davemloft.net>