sfc: report supported link speeds on SFP connections

7000-series SFC NICs connected with an SFP+ module currently fail to
report any supported link speeds.

Reported-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Reviewed-by: Jarod Wilson <jarod@redhat.com>
Tested-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>