sfc: Fix SFE4002 initialisation

From: Steve Hodgson <shodgson@solarflare.com>

Commit 357d46a1 "sfc: QT202x: Remove
unreliable MMD check at initialisation" broke initialisation of the
SFE4002.  efx_mdio_reset_mmd() returns a positive value rather than 0
on success.  The above commit causes this value to be propagated up
by qt202x_reset_phy(), which is treated as a failure by its callers.
Change qt202x_reset_phy() to return 0 if successful.

The PCI layer treats >0 as "fail, but please call remove() anyway",
which means that unloading the driver would cause a crash.  Add a
WARN_ON() on the failure path of efx_pci_probe() to provide early
warning if there are any other cases where we do this.
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/sfc/efx.c

@@ -2284,6 +2284,7 @@ static int __devinit efx_pci_probe(struct pci_dev *pci_dev,
  fail2:
 	efx_fini_struct(efx);
  fail1:
+	WARN_ON(rc > 0);
 	EFX_LOG(efx, "initialisation failed. rc=%d\n", rc);
 	free_netdev(net_dev);
 	return rc;

drivers/net/sfc/qt202x_phy.c

@@ -320,7 +320,7 @@ static int qt202x_reset_phy(struct efx_nic *efx)
 
 	falcon_board(efx)->type->init_phy(efx);
 
-	return rc;
+	return 0;
 
  fail:
 	EFX_ERR(efx, "PHY reset timed out\n");