cxl/cdat: Fix header sum value in CDAT checksum

cxl/cdat: Fix header sum value in CDAT checksum

The addition of the DCD support for CXL type-3 devices extended the CDAT
table large enough that the checksum being returned was incorrect.[1]

This was because the checksum value was using the header length field
rather than each of the 4 bytes of the length field.  This was
previously not seen because the length of the CDAT data was less than
256 thus resulting in an equivalent checksum value.

Properly calculate the checksum for the CDAT header.

[1] https://lore.kernel.org/all/20231116-fix-cdat-devm-free-v1-1-b148b40707d7@intel.com/

Fixes: aba578bdace5 ("hw/cxl/cdat: CXL CDAT Data Object Exchange implementation")
Cc: Huai-Cheng Kuo <hchkuo@avery-design.com.tw>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20240126120132.24248-5-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
(cherry picked from commit 64fdad5e67587e88c2f1d8f294e89403856a4a31)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>