sfc: Introduce and use MCDI_CTL_SDU_LEN_MAX_V1 macro for Siena-specific code

The MCDI version 2 protocol supports larger payloads, but will
not be implemented on Siena.

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

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

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