V4L/DVB (9631): Make s2api work for ATSC support

ATSC should be considered a legacy delivery system, or else fields such as
p->u.vsb.modulation do not get populated (resulting in set_frontend failures)

Cc: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Devin Heitmueller <devin.heitmueller@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>