sfc: add sysfs entry to control MCDI tracing

MCDI tracing is enabled per-function with a sysfs file
    /sys/class/net/<NET_DEV>/device/mcdi_logging

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/ethernet/sfc/mcdi.c b/drivers/net/ethernet/sfc/mcdi.c
index 31eda325..dde7f90 100644
--- a/drivers/net/ethernet/sfc/mcdi.c
+++ b/drivers/net/ethernet/sfc/mcdi.c
@@ -188,7 +188,7 @@
 	}
 
 #ifdef CONFIG_SFC_MCDI_LOGGING
-	if (!WARN_ON_ONCE(!buf)) {
+	if (mcdi->logging_enabled && !WARN_ON_ONCE(!buf)) {
 		int bytes = 0;
 		int i;
 		/* Lengths should always be a whole number of dwords, so scream
@@ -274,7 +274,7 @@
 	}
 
 #ifdef CONFIG_SFC_MCDI_LOGGING
-	if (!WARN_ON_ONCE(!buf)) {
+	if (mcdi->logging_enabled && !WARN_ON_ONCE(!buf)) {
 		size_t hdr_len, data_len;
 		int bytes = 0;
 		int i;