V4L/DVB (13899): gspca - all subdrivers: Make control descriptors constant.

The ctrls field of struct sd_desc is declared as const
in gspca.h. It is worth to initialize the content also with
constant values.

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