sfc: Make MCDI independent of Siena

Move the lowest layer (transport) of the current MCDI code to
per-NIC-type operations.

Introduce a new structure and efx_nic member for MCDI-specific data.

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

diff --cc drivers/net/ethernet/sfc/mcdi.c
Simple merge

diff --cc drivers/net/ethernet/sfc/mcdi.h
Simple merge

diff --cc drivers/net/ethernet/sfc/net_driver.h
Simple merge

diff --cc drivers/net/ethernet/sfc/nic.h
Simple merge

diff --cc drivers/net/ethernet/sfc/siena.c
Simple merge