sfc: Allow efx_mcdi_rpc to be called in two parts

For NIC/System time synchonisation efx_mcdi_rpc needs to be split in
efx_mcdi_rpc_start and efx_mcdi_rpc_finish operations.

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