PCI: Use PCI_STD_NUM_BARS instead of 6

pci_read_bases() is given literal 6 that means PCI_STD_NUM_BARS.  Replace
the literal with the define to annotate the code better.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Link: https://patch.msgid.link/20250416100239.6958-1-ilpo.jarvinen@linux.intel.com