sfc: return errors from efx_mcdi_set_id_led, and de-indirect

W=1 warnings indicated that 'rc' was unused in efx_mcdi_set_id_led();
 change the function to return int instead of void and plumb the rc
 through the caller efx_ethtool_phys_id().
Since (post-Falcon) all sfc NICs use MCDI for this, there's no point in
 indirecting through a nic_type method, so remove that and just call
 efx_mcdi_set_id_led() directly.

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