fw_cfg: fix driver remove

On driver remove(), all objects created during probe() should be
removed, but sysfs qemu_fw_cfg/rev file was left. Also reorder
functions to match probe() error cleanup code.

Cc: stable@vger.kernel.org
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>