x86/PCI: Clear host bridge aperture struct resource

Use kzalloc() so the struct resource doesn't contain garbage in
fields we don't initialize.

[bhelgaas: changelog]
Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: x86@kernel.org