V4L/DVB (8791): v4l2-dev: do not clear the driver_data field

The driver_data field of the device structure was zeroed, thus losing
this information. Many drivers set this up before calling
video_device_register, so we have to ensure that it isn't lost.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>