PCI: Split out next_ari_fn() from next_fn()

PCI: Split out next_ari_fn() from next_fn()

In commit b1bd58e448f2 ("PCI: Consolidate "next-function" functions") the
next_fn() function subsumed the traditional and ARI-based next function
determination. This got rid of some needlessly complex function pointer
handling but also reduced the separation between these very different
methods of finding the next function. With the next_fn() cleaned up a bit
we can re-introduce this separation by moving out the ARI handling while
sticking with direct function calls.

Link: https://lore.kernel.org/r/20220628143100.3228092-3-schnelle@linux.ibm.com
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>