PCI: endpoint: Replace magic number '6' by PCI_STD_NUM_BARS

Replace the constant "6" by PCI_STD_NUM_BARS, as defined in
include/uapi/linux/pci_regs.h:

  #define PCI_STD_NUM_BARS       6       /* Number of standard BARs */

Link: https://lore.kernel.org/r/20241212162547.225880-1-rick.wertenbroek@gmail.com


Signed-off-by: Rick Wertenbroek <rick.wertenbroek@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>