m68k: cmpxchg: Fix return value for default case in __arch_xchg()

The return value of __invalid_xchg_size() is assigned to tmp instead of
the return variable x. Assign it to x instead.

Fixes: 2501cf76 ("m68k: Fix xchg/cmpxchg to fail to link if given an inappropriate pointer")
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/20240702034116.140234-2-thorsten.blum@toblux.com


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