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>