ixgbe: fix probing of multi-port devices with one MDIO

ixgbe: fix probing of multi-port devices with one MDIO

Ian reports that after upgrade from v5.8.14 to v5.9 only one
of his 4 ixgbe netdevs appear in the system.

Quoting the comment on ixgbe_x550em_a_has_mii():
 * Returns true if hw points to lowest numbered PCI B:D.F x550_em_a device in
 * the SoC.  There are up to 4 MACs sharing a single MDIO bus on the x550em_a,
 * but we only want to register one MDIO bus.

This matches the symptoms, since the return value from
ixgbe_mii_bus_init() is no longer ignored we need to handle
the higher ports of x550em without an error.

Fixes: 09ef193fef7e ("net: ethernet: ixgbe: check the return value of ixgbe_mii_bus_init()")
Reported-by: Ian Kumlien <ian.kumlien@gmail.com>
Tested-by: Ian Kumlien <ian.kumlien@gmail.com>
Acked-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20201016232006.3352947-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>