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/Kconfig b/drivers/net/ethernet/sfc/Kconfig
index c4ba42b..4dd92b7 100644
--- a/drivers/net/ethernet/sfc/Kconfig
+++ b/drivers/net/ethernet/sfc/Kconfig
@@ -43,4 +43,5 @@
 	---help---
 	  This enables support for tracing of MCDI (Management-Controller-to-
 	  Driver-Interface) commands and responses, allowing debugging of
-	  driver/firmware interaction.
+	  driver/firmware interaction.  The tracing is actually enabled by
+	  a sysfs file 'mcdi_logging' under the PCI device.