powerpc/powernv: Fix potential zero devisor

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: Anton Blanchard <anton@samba.org>
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>