Add the constant valid length of MI command.

v2: Add F_VAL_CONST flag. (Zhenyu Wang)
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Gao, Fred <fred.gao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>