usb/storage: clear csw on reset

Stale data in csw (specifically residue) can confuse the state machine
and allows the guest trigger an assert().  So clear csw on reset to
avoid this happening in case the guest resets the device in the middle
of a request.

Buglink: https://bugs.launchpad.net/qemu/+bug/1523811


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210312094954.796799-1-kraxel@redhat.com>