V4L/DVB: dvb: return -ENOMEM if kzalloc failed in dvb_usb_device_init()

If in a cold state and the download succeeded ret is zero, but we
should return -ENOMEM.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>