V4L/DVB (11393): radio-si470x: fix possible bug with freeing memory order

Patch fixes cleanup procedure in si470x_usb_driver_probe. Add new label
err_video and change order of freeing memory.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>