sfc: Reset driver's MAC stats after MC reboot seen

If the MC reboots then the stats it reports to us will have been
reset.  We need to reset ours to get efx_update_diff_stat() working
properly.

(Ideally we would maintain stats across the reboot, but as this should
only happen immediately after a firmware upgrade it's not really worth
the trouble.)
Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>

drivers/net/ethernet/sfc/mcdi.c

@@ -207,7 +207,9 @@ static int efx_mcdi_poll(struct efx_nic *efx)
 	return 0;
 }
 
-/* Test and clear MC-rebooted flag for this port/function */
+/* Test and clear MC-rebooted flag for this port/function; reset
+ * software state as necessary.
+ */
 int efx_mcdi_poll_reboot(struct efx_nic *efx)
 {
 	unsigned int addr = FR_CZ_MC_TREG_SMEM + MCDI_STATUS(efx);
@@ -223,6 +225,11 @@ int efx_mcdi_poll_reboot(struct efx_nic *efx)
 	if (value == 0)
 		return 0;
 
+	/* MAC statistics have been cleared on the NIC; clear our copy
+	 * so that efx_update_diff_stat() can continue to work.
+	 */
+	memset(&efx->mac_stats, 0, sizeof(efx->mac_stats));
+
 	EFX_ZERO_DWORD(reg);
 	efx_writed(efx, &reg, addr);