hw/cxl/cxl_device: Replace magic number in CXLError definition

hw/cxl/cxl_device: Replace magic number in CXLError definition

Replace the magic number 32 with CXL_RAS_ERR_HEADER_NUM for better code
readability and maintainability.

Signed-off-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>