If there're no PHBs under P5IOC2 HUB device tree node, we should
bail early to avoid zero devisor and allocating TCE tables.
Reported-by: NAnton Blanchard <anton@samba.org>
Signed-off-by: NGavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>