[media] em28xx-v4l: simplify em28xx_v4l2_open() by using v4l2_fh_open()

Instead of calling

...
struct v4l2_fh *fh = kzalloc(sizeof(*fh), GFP_KERNEL);
filp->private_data = fh;
v4l2_fh_init(fh, vdev);
v4l2_fh_add(fh);
...

simply use function v4l2_fh_open() which does all of these calls for us.

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