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