PCI: generic: Allocate config space windows after limiting bus number range

The number of config space windows allocated for the host bridge depends on
how many bus numbers are below the bridge.  Instead of first allocating the
windows and then limiting the bus resource, this patch reshuffles the code
so that if any limitation is applied to the bus resource, it is taken into
account in the windows allocation.

[bhelgaas: changelog]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Will Deacon <will.deacon@arm.com>