powerpc/pseries/eeh: Make pseries_pcibios_bus_add_device() static

powerpc/pseries/eeh: Make pseries_pcibios_bus_add_device() static

pseries_pcibios_bus_add_device() is a local routine defining the
pcibios_bus_add_device() handler of the pseries machine in
eeh_pseries_init(). It doesn't need to be external.

It fixes this W=1 compile error:

../arch/powerpc/platforms/pseries/eeh_pseries.c:46:6: error: no previous prototype for ‘pseries_pcibios_bus_add_device’ [-Werror=missing-prototypes]
   46 | void pseries_pcibios_bus_add_device(struct pci_dev *pdev)
      |      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fixes: dae7253f9f78 ("powerpc/pseries: Add pseries SR-IOV Machine dependent calls")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210104143206.695198-4-clg@kaod.org