sfc: Add support for MCDI v2

MCDI v2 adds a second header dword with wider command and length
fields.  It also defines extra error codes.

Change the fallback error number for unknown MCDI error codes from EIO
to EPROTO.  EIO is treated as indicating the MCDI transport has failed
and we need to reset the function, which is rather drastic.

v2 error codes and lengths don't fit into completion events, so for a
v2-capable transport, always read the response header rather then
using the event fields.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>