[media] gscpa_stv06xx: Make sd_desc const

Now that stv06xx is using the control framework it is no longer necessary
to have a (non const) copy of sd_desc inside the sd specific data struct.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>