xen/pt: Fixup addr validation in xen_pt_pci_config_access_check

xen_pt_pci_config_access_check checks if addr >= 0xFF. 0xFF is a valid
address and should not be ignored.

Signed-off-by: Anoob Soman <anoob.soman@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
