[media] em28xx: move struct v4l2_clk *clk from struct em28xx to struct v4l2

The current clock definition applies only to the V4L2 side of the
driver. Move its struct pointer to the v4l2_dev, where it belongs.

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>