V4L/DVB (3313): FIX: Check if FW was downloaded or not + new firmware file

- When a firmware was downloaded dvb_usb_device_init returns NULL for the
  dvb_usb_device, then nothing should be done with that pointer and device,
  because it will re-enumerate.
- A new firmware should be used with digitv devices. 
- It should make "slave"-devices work and others, too.

Signed-off-by: Patrick Boettcher <pb@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>