vidioc_enum_fmt_vid_overlay() did nothing but call
vidioc_enum_fmt_vid_cap(), so just make
saa7146_video_ioctl_ops.vidioc_enum_fmt_vid_overlay point to
vidioc_enum_fmt_vid_cap() and get ride of vidioc_enum_fmt_vid_overlay().
Have gparm use v4l2_video_std_frame_period to fill in the frame period
instead of just assuming PAL.