[media] drivers/media/usb/dvb-usb-v2: constify mxl111sf_demod_config structure

The mxl111sf_demod_config structure is never modified, so declare it
as const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Reviewed-by: Michael Ira Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>