PCI/IOV: Clarify error message for unbound devices

Be more verbose to disambiguate the error case when trying to configure
SR-IOV with no driver bound vs. a driver that does not implement the
.sriov_configure() callback.

Link: https://lore.kernel.org/r/20210327175140.682708-1-mdf@kernel.org


Reported-by: Brian Foley <bpfoley@google.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>