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

arch/m68k/include/asm/cmpxchg.h

@@ -32,7 +32,7 @@ static inline unsigned long __arch_xchg(unsigned long x, volatile void * ptr, in
 		x = tmp;
 		break;
 	default:
-		tmp = __invalid_xchg_size(x, ptr, size);
+		x = __invalid_xchg_size(x, ptr, size);
 		break;
 	}