powerpc/powernv: Exclude root bus in pnv_pci_reset_secondary_bus()

The function pnv_pci_reset_secondary_bus() is called like below.
It's impossible for call the function on root bus. So it's safe
to remove the root bus case in the function. No functional changes
introduced.

   pci_parent_bus_reset() / pci_bus_reset() / pci_try_reset_bus()
   pci_reset_bridge_secondary_bus()
   pcibios_reset_secondary_bus()
   pnv_pci_reset_secondary_bus()

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>