PCI: honor child buses add_size in hot plug configuration

git commit c8adf9a3
    "PCI: pre-allocate additional resources to devices only after
	successful allocation of essential resources."

fails to take into consideration the optional-resources needed by children
devices while calculating the optional-resource needed by the bridge.

This can be a problem on some setup. For example, if a hotplug bridge has 8
children hotplug bridges, the bridge should have enough resources to accomodate
the hotplug requirements for each of its children hotplug bridges.  Currently
this is not the case.

This patch fixes the problem.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Reviewed-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>