‘value’may be used uninitialized in this function `kbd_io`

When build with gcc (GCC) 8.4.1, i got a error
```
hw/i8042.c: In function ‘kbd_io’:
hw/i8042.c:153:19: error: ‘value’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
   state.write_cmd = val;
   ~~~~~~~~~~~~~~~~^~~~~
hw/i8042.c:298:5: note: ‘value’ was declared here
  u8 value;
     ^~~~~
cc1: all warnings being treated as errors

```