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>