V4L/DVB (9179): S2API: frontend.h cleanup

Reviewing the code briefly and saw this.
You can't change more than DTV_IOCTL_MAX_MSGS at once, not 16.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Steven Toth <stoth@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>