V4L/DVB: video_ioctl2: do not replace arg with NULL for _IO() ioctls

If the ioctl was defined without direction (e.g. _IO('o', 25)), then
the arg as passed to vidioc_default was NULL instead of the original
argument.

Several ioctls in e.g. include/linux/dvb/video.h and audio.h use this type
of ioctl to pass simple numerical values to the driver.

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