powerpc/powernv: Check primary PHB through ID

The index of one specific PCI controller (struct pci_controller::
global_number) can tell that it's primary one or not. So we needn't
additional variable for that and just remove it.

Signed-off-by: Gavin Shan <shangw@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>