[SeaBIOS] hw/usb-msc.c: Branch condition evaluates to a garbage value

[Paul Menzel]

Dear SeaBIOS developers,

Building the attached configuration with GCC 5.3.0 and the static 
analyzer scan-build shipped with Clang 3.8, the following error is 
reported in `src/hw/usb-msc.c` in line 110.

The static analyzer seems to think that the struct member is not 
initialized. But I guess it’s done in the line below, and therefore a 
false positive?

```
ret = usb_msc_send(udrive_gf, USB_DIR_IN
                        , MAKE_FLATPTR(GET_SEG(SS), &csw), sizeof(csw));
```

Thanks,

Paul