r8169: check for PCI read error in probe

Check whether first PCI read returns 0xffffffff. Currently, if this is
the case, the user sees the following misleading message:
unknown chip XID fcf, contact r8169 maintainers (see MAINTAINERS file)

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/75b54d23-fefe-2bf4-7e80-c9d3bc91af11@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>