[media] stk-webcam: don't use private_data, use video_drvdata

file->private_data is needed to store the pointer to struct v4l2_fh.
So use video_drvdata to get hold of the stk_camera struct.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Arvydas Sidorenko <asido4@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>