sfc_ef100: PHY probe stub

We can't actually do the MCDI to probe it fully until we have working
 MCDI, which comes later, but we need efx->phy_data to be allocated so
 that when we get MCDI events the link-state change handler doesn't
 NULL-dereference.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>