i2o: Use pci_bus_alloc_resource(), not allocate_resource() directly

Convert i2o_res_alloc() to use pci_bus_alloc_resource() rather than
pci_find_parent_resource() and allocate_resource().  We don't have a
resource to start with, so pci_find_parent_resource() can't do anything
useful: a bus may have several memory resources available, so there might
be several possible parents.  This is more likely on root buses because
host bridges may have any number of apertures.

I'm pretty sure this didn't work in the first place because it passed
size == min == max to allocate_resource().  The min and max parameters are
constraints on the *addresses* of the resource, not on its size, so I think
it was impossible for allocate_resource() to succeed.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>