cxl/pci: Use pci core's DVSEC functionality

Reduce maintenance burden of DVSEC query implementation by using the
centralized PCI core implementation.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
[djbw: kill cxl_pci_dvsec()]
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/163379788528.692348.11581080806976608802.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>