m68k: fix flatmem memory model setup

Detected a broken boot on mcf54415, likely introduced from

commit 4bfc848e


("m68k/mm: enable use of generic memory_model.h for !DISCONTIGMEM")

Fix ARCH_PFN_OFFSET to be a pfn.

Signed-off-by: Angelo Dureghello <angelo@kernel-space.org>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>