m68k: correct setting of struct user.u_ar0

Commit 6e16d89b

 ("Sanitize the type of
struct user.u_ar0") forgot to change the m68k setting code, causing the
following compiler warning:

    arch/m68k/kernel/process.c:338: warning: assignment makes integer from pointer without a cast

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>