pci/bridge: fix pci_bridge_reset()

The lower bits of base/limit registers is RO and shouldn't be zero
cleared on reset. This patch fixes it.
In fact, the default value of base/limit registers aren't specified
in the spec. And some bridges disable forwarding on reset instead of
zeroing base/limit registers.
So introduce one function to disable bridge forwarding so that
such bridges can use it. It will be used later.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
