[pxe] Improve pxe_undi debug messages

The PXE debugging messages have remained pretty much unaltered since
Etherboot 5.4, and are now difficult to read in comparison to most of
the rest of gPXE.

Bring the pxe_undi debug messages up to normal gPXE standards.

src/include/gpxe/pci.h

@@ -321,6 +321,7 @@ struct pci_driver {
 #define PCI_DEVFN( slot, func )		( ( (slot) << 3 ) | (func) )
 #define PCI_SLOT( devfn )		( ( (devfn) >> 3 ) & 0x1f )
 #define PCI_FUNC( devfn )		( (devfn) & 0x07 )
+#define PCI_BUS( busdevfn )		( (busdevfn) >> 8 )
 #define PCI_BUSDEVFN( bus, devfn )	( ( (bus) << 8 ) | (devfn) )
 
 #define PCI_BASE_CLASS( class )		( (class) >> 16 )