V4L/DVB (7946): videodev: small fixes for VIDIOC_G_FREQUENCY and VIDIOC_G_FMT

__video_do_ioctl incorrectly zeroed the tuner field of v4l2_frequency and
did not zero the full fmt union of v4l2_format.

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