sfc: add tracing of MCDI commands

MCDI tracing is conditional on CONFIG_SFC_MCDI_LOGGING, which is enabled
 by default.

Each MCDI command will produce a console line like
    sfc dom🚌dev:fn ifname: MCDI RPC REQ: xxxxxxxx [yyyyyyyy...]
where xxxxxxxx etc. are the raw MCDI payload in 32-bit hex chunks.
The response will then produce a similar line with "RESP" instead of "REQ",
 and containing the MCDI response payload (if any).

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sfc/mcdi.h

@@ -58,6 +58,7 @@ enum efx_mcdi_mode {
  *	enabled
  * @async_list: Queue of asynchronous requests
  * @async_timer: Timer for asynchronous request timeout
+ * @logging_buffer: buffer that may be used to build MCDI tracing messages
  */
 struct efx_mcdi_iface {
 	struct efx_nic *efx;
@@ -74,6 +75,9 @@ struct efx_mcdi_iface {
 	spinlock_t async_lock;
 	struct list_head async_list;
 	struct timer_list async_timer;
+#ifdef CONFIG_SFC_MCDI_LOGGING
+	char *logging_buffer;
+#endif
 };
 
 struct efx_mcdi_mon {