V4L/DVB: v4l2-common: Move v4l2_find_nearest_format from videodev2.h to v4l2-common.h

This function is an internal API and belongs in v4l2-common.h, not
videodev.h. The return pointer and probe argument should be const as well.

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