[POWERPC] Fix bogus test for unassigned PCI resources

A bogus test for unassigned resources that came from our 32-bit
PCI code ended up being "merged" by my previous patch series,
breaking some 64-bit setups where devices have legal resources
ending at 0xffffffff.

This fixes it by completely changing the test.  We now test for
res->start == 0, as the generic code expects, and we also only
do so on platforms that don't have the PPC_PCI_PROBE_ONLY flag
set, as there are cases of pSeries and iSeries where it could
be a valid value and those can't reassign devices.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>