sfc: make firmware-variant printing a nic_type function

Instead of having efx_mcdi_print_fwver() look at efx_nic_rev and
 conditionally poke around inside ef10-specific nic_data, add a new
 efx->type->print_additional_fwver() method to do this work.

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>