V4L/DVB (9315): s5h1411: Skip reconfiguring demod modulation if already at the desired modulation

If we are already at the desired modulation, there is no need to reconfigure
the demod (at a tuning time cost)

Note that this change revealed that although the datasheet says the demod
starts out in VSB-8 mode, the first tuning was failing consistently unless
we went through the work of setting the registers.  So add a field to denote
this case so we always do the enable_frontend call, even if the first tuning
request is for VSB-8.

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