sfc: suppress some MCDI error messages in PTP

Also, remove a needless netif_err() from efx_ptp_update_stats() - if the
 MCDI fails it'll print its own error message, we don't need another that
 adds no information.

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