V4L/DVB (13898): gspca - some subdrivers: Make sd_desc const.

The function callbacks in sd_desc are defined at compile time and
they do not change at runtime. Make the sd_desc initializations const.

Signed-off-by: Márton Németh <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>