m68k: __pa(): cast arg to long

Fixes this:

drivers/char/mem.c: In function 'mmap_kmem':
drivers/char/mem.c:342: warning: cast to pointer from integer of different size

by doing what other archtiectures do.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>