xen/arm: pci: fix -Wtype-limits warning in pci-host-common.c

xen/arm: pci: fix -Wtype-limits warning in pci-host-common.c

When building with EXTRA_CFLAGS_XEN_CORE="-Wtype-limits", we observe the
following warning:

arch/arm/pci/pci-host-common.c: In function ‘pci_host_common_probe’:
arch/arm/pci/pci-host-common.c:238:26: warning: comparison is always false due to limited range of data type [-Wtype-limits]
  238 |     if ( bridge->segment < 0 )
      |                          ^

This is due to bridge->segment being an unsigned type. Fix it by introducing a
new variable of signed type to use in the condition.

Fixes: 6ec9176d94ae ("xen/arm: PCI host bridge discovery within XEN on ARM")
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Rahul Singh <rahul.singh@arm.com<mailto:rahul.singh@arm.com>>