m68k: Make thread_info.h usable from assembly.

[Geert] <asm/thread_info_mm.h> pulls in <asm/current.h>, which contains C only.
So the include must be moved inside #ifndef __ASSEMBLY__.

Signed-off-by: Tim Abbott <tabbott@ksplice.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>