Cast type before the left shift

Building ipmitool with UBSAN and I got:
```
...
xxx/ipmitool/include/ipmitool/helper.h:191:14: runtime error: left shift of 160
by 24 places cannot be represented in type 'int'
    #0 0x55bddaa56f11 in ipmi32toh
       xxx/ipmitool/include/ipmitool/helper.h:191:14
```

Tested: with this, I tested ipmitool again and the issue disappeared.

Resolved: ipmitool/ipmitool#352
Signed-off-by: Tom Tung <shes050117@gmail.com>