inline asm: return value is zero if four arguments passed

MicroPython v1.6-223-g77f85db on 2016-03-15; PYBv1.0 with STM32F405RG

``` python
@micropython.asm_thumb
def foo(r0, r1, r2, r3):
    movw(r0, 25)
```

```
>>> foo(1,2,3,4)
0
>>> 
```

The fault only occurs if the function uses four arguments.
