[media] dvb_usb_v2: return the download ret in dvb_usb_download_firmware

The first being this patch, no return value from dvb_usb_download_firmware
causes system wide dead lock with COLD disconnect as system attempts to continue
to warm state.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>