sfc: Fix division by zero when using one RX channel and no SR-IOV

If RSS is disabled on the PF (efx->n_rx_channels == 1) we try to set
up the indirection table so that VFs can use it, setting
efx->rss_spread = efx_vf_size(efx).  But if SR-IOV was disabled at
compile time, this evaluates to 0 and we end up dividing by zero when
initialising the table.

I considered changing the fallback definition of efx_vf_size() to
return 1, but its value is really meaningless if we are not going to
enable VFs.  Therefore add a condition of efx_sriov_wanted(efx) in
efx_probe_interrupts().
Signed-off-by: NBen Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/efx.c

@@ -1349,7 +1349,7 @@ static int efx_probe_interrupts(struct efx_nic *efx)
 	}
 
 	/* RSS might be usable on VFs even if it is disabled on the PF */
-	efx->rss_spread = (efx->n_rx_channels > 1 ?
+	efx->rss_spread = ((efx->n_rx_channels > 1 || !efx_sriov_wanted(efx)) ?
 			   efx->n_rx_channels : efx_vf_size(efx));
 
 	return 0;