media: dvb-frontends: tda18271c2dd: Constify static structs

Constify a number of static structs that were not modified. In order to
be able to do that, const input arguments to a couple of functions that
not modify their argument. This allows the compiler to put them in
read-only memory, resulting in about 6k memory being read-only, and the
resulting module is 400 bytes smaller.

Before:
   text    data     bss     dec     hex filename
  15492    6873       0   22365    575d drivers/media/dvb-frontends/tda18271c2dd.ko

After:
   text    data     bss     dec     hex filename
  21021     897       0   21918    559e drivers/media/dvb-frontends/tda18271c2dd.ko

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>