powerpc/powernv/pci: Add pci_bus_to_pnvhb() helper

Add a helper to go from a pci_bus structure to the pnv_phb that hosts
that bus. There's a lot of instances of the following pattern:

	struct pci_controller *hose = pci_bus_to_host(pdev->bus);
	struct pnv_phb *phb = hose->private_data;

Without any other uses of the pci_controller inside the function. This
is hard to read since it requires you to memorise the contents of the
private data fields and kind of error prone since it involves blindly
assigning a void pointer. Add a helper to make it more concise and
explicit.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200722065715.1432738-1-oohall@gmail.com