[media] ivtv: Fix sparse warning regarding a user pointer in ivtv_write_vbi_from_user()

Fix the first, botched attempt at preventing direct use of a user pointer in
ivtv_write_vbi().

Reported-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>