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 --cc drivers/net/ethernet/sfc/Kconfig
Simple merge

diff --cc drivers/net/ethernet/sfc/efx.c
Simple merge

diff --cc drivers/net/ethernet/sfc/mcdi.c
Simple merge

diff --cc drivers/net/ethernet/sfc/mcdi.h
Simple merge