V4L/DVB: s2255drv: removes kref tracking and videodev parent

kref replaced as suggested in code review
uses atomic variable to track when it is ok to delete device
removes setting of video device parent, which is now
handled in v4l2_device.c

Signed-off-by: Dean Anderson <linux-dev@sensoray.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>