[media] ttusb-dec: constify ttusbdecfe_config structure

The ttusbdecfe_config structure is never modified, so declare it
as const.

Other references to this structure type were already declared as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>