sfc: Use MCDI RX_BAD_FCS_PKTS count as MAC rx_bad count

Calculating rx_bad as rx_packets - rx_good is unnecessary and
incorrect, since rx_good does not include control frames (e.g.
pause frames) and rx_packets does.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>