net/sfc: support MCDI proxy

The patch is to add support for MCDI proxy which comes in
useful, particularly, while running over VF: few commands
will normally fail with EPERM, but in some cases the host
driver (i.e. running over the corresponding PF, typically,
within a hypervisor) may set itself as a proxy to conduct
authorization for the commands coming from VFs; these are
forwarded to the corresponding access control application
which may decline or approve authorization by replying to
the requests; all in all, the guest driver has to process
the replies forwarded back by the firmware MC in order to
give up gracefully (by setting return code which could be
understood by 'libefx') or re-issue the original commands

Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Signed-off-by: Andrew Rybchenko <arybchenko@solarflare.com>
Reviewed-by: Andy Moreton <amoreton@solarflare.com>