PCI: Add dword #defines for Bus Number + Secondary Latency Timer

PCI: Add dword #defines for Bus Number + Secondary Latency Timer

uapi/linux/pci_regs.h defines Primary/Secondary/Subordinate Bus Numbers
and Secondary Latency Timer (PCIe r7.0, sec. 7.5.1.3) as byte register
offsets, but in practice the code may read/write the entire dword. In the
lack of #defines to handle the dword fields, the code ends up using
literals which are not as easy to read.

Add dword field masks for the Bus Number and Secondary Latency Timer
fields and use them in probe.c.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: squash new #defines and uses together]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20251219174036.16738-21-ilpo.jarvinen@linux.intel.com
Link: https://patch.msgid.link/20251219174036.16738-22-ilpo.jarvinen@linux.intel.com