sfc: Implement asynchronous MCDI requests

This will allow use of MCDI from the data path, in particular for
accelerated RFS.

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

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