pci: fix handling of PCI bridges with subordinate bus number 0xff

Bus number 0xff is valid according to the PCI spec. Using u8 typed sub_bus
and assigning 0xff to it will result in the following loop getting stuck.

    for ( ; sec_bus <= sub_bus; sec_bus++ ) {...}

Just change its type to unsigned int similarly to what is already done in
dmar_scope_add_buses().

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
master commit: 9c3b9800e2
master date: 2021-09-28 16:04:50 +0200

xen/drivers/passthrough/pci.c

@@ -375,8 +375,7 @@ static struct pci_dev *alloc_pdev(struct pci_seg *pseg, u8 bus, u8 devfn)
     /* update bus2bridge */
     switch ( pdev->type = pdev_type(pseg->nr, bus, devfn) )
     {
-        u16 cap;
-        u8 sec_bus, sub_bus;
+        unsigned int cap, sec_bus, sub_bus;
 
         case DEV_TYPE_PCIe2PCI_BRIDGE:
         case DEV_TYPE_LEGACY_PCI_BRIDGE:
@@ -443,7 +442,7 @@ static void free_pdev(struct pci_seg *pseg, struct pci_dev *pdev)
     /* update bus2bridge */
     switch ( pdev->type )
     {
-        uint8_t sec_bus, sub_bus;
+        unsigned int sec_bus, sub_bus;
 
         case DEV_TYPE_PCIe2PCI_BRIDGE:
         case DEV_TYPE_LEGACY_PCI_BRIDGE: