sfc: Fold falcon_probe_nic_variant() into falcon_probe_nic()

falcon_probe_nic_variant() does a lot less than it used to, and a
lot less than it claims to.  Fold the remainder into its caller.

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