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>
diff --git a/drivers/net/sfc/siena.c b/drivers/net/sfc/siena.c
index 3fab030..9f53680 100644
--- a/drivers/net/sfc/siena.c
+++ b/drivers/net/sfc/siena.c
@@ -450,7 +450,7 @@
 				    mac_stats->rx_bad_bytes);
 	MAC_STAT(rx_packets, RX_PKTS);
 	MAC_STAT(rx_good, RX_GOOD_PKTS);
-	mac_stats->rx_bad = mac_stats->rx_packets - mac_stats->rx_good;
+	MAC_STAT(rx_bad, RX_BAD_FCS_PKTS);
 	MAC_STAT(rx_pause, RX_PAUSE_PKTS);
 	MAC_STAT(rx_control, RX_CONTROL_PKTS);
 	MAC_STAT(rx_unicast, RX_UNICAST_PKTS);