PCI: Remove redundant PCI_VSEC_HDR and PCI_VSEC_HDR_LEN_SHIFT

Remove duplicate macro PCI_VSEC_HDR and its related macro
PCI_VSEC_HDR_LEN_SHIFT from pci_regs.h to avoid redundancy and
inconsistencies. Update VFIO PCI code to use PCI_VNDR_HEADER and
PCI_VNDR_HEADER_LEN() for consistent naming and functionality.

These changes aim to streamline header handling while minimizing impact,
given the niche usage of these macros in userspace.

Link: https://lore.kernel.org/r/20241216013536.4487-1-zhangdongdong@eswincomputing.com


Signed-off-by: Dongdong Zhang <zhangdongdong@eswincomputing.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>