[media] cx24120: constify static data

Use const on the static data, as gcc may optimize better the
code. Also, would prevent that some code would override the
data there.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>