V4L/DVB (9173): S2API: Remove the hardcoded command limit during validation

This means that when developers add new commands then they'll be see
the DTV_MAX_COMMAND define and will be more likely to modify it, without
having to modify the command validation code.

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