V4L/DVB: drivers/media/radio/si470x/radio-si470x-usb.c fix use after free

In si470x_usb_driver_disconnect() radio->disconnect_lock is accessed
after it is freed. This fixes the problem.

Coverity CID: 2530

Signed-off-by: Darren Jenkins <darrenrjenkins@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>