m68k: fix __page_to_pfn for a const struct page argument

Fixes fallout due to the removal of the cast in commit aa462abe


("mm: fix __page_to_pfn for a const struct page argument")

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>