m68k: mm: Fix flatmem memory model setup

m68k: mm: Fix flatmem memory model setup

Detected a broken boot on mcf54415, likely introduced from

commit 4bfc848e0981
("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>
Link: https://lore.kernel.org/r/20210228190828.392974-1-angelo@kernel-space.org
Fixes: 4bfc848e0981fcd3 ("m68k/mm: enable use of generic memory_model.h for !DISCONTIGMEM")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>