[media] ivtv: Return EFAULT when copy_from_user() fails in ivtv_write_vbi_from_user()

If write() on a VBI device node fails due to a bad buffer pointer from
userspace, we should notify the application properly with EFAULT, per the
V4L2 API spec.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>