powernv/pci: Use pnv_phb as the private data for debugfs entries

Use the pnv_phb structure as the private data pointer for the debugfs
files.  This lets us delete some code and an open-coded use of
hose->private_data.

Signed-off-by: Oliver O'Halloran <oohall@gmail.com>
Reviewed-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190912052945.12589-1-oohall@gmail.com