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:bus: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>