powerpc/iseries: Fix possible null pointer dereference in iSeries_pcibios_fixup_resources

I don't know if this is a right fix for the problem
since of_get_property can return NULL.
Since iseries_device_information is used only for informational purpose,
we can skip this function without valid HvSubBusNumber number.

Signed-off-by: Denis Kirjanov <dkirjanov@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>