m68k: Fix asm constraints for atomic_sub_and_test() and atomic_add_negative()

Recently, we started seeing this on allmodconfig builds:

  CC      mm/memcontrol.o
{standard input}: Assembler messages:
{standard input}:4076: Error: operands mismatch -- statement `subl 12(%fp),170(%a0)' ignored

Correct the asm constraint, like done for m68knommu.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>