sfc: Fix ordering of device registration and initial netif_carrier_off()

We must call netif_carrier_off() after the device is registered, not
before, to set the operational state and user-space IFF_RUNNING flag
correctly.

Since we don't want observers to see an intermediate state, open-code
register_netdev() and add efx_update_name() and netif_carrier_off()
into the locked region.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>