[media] Make nchg variable signed because the code compares this variable against negative values

The sonixj driver compares the value for nchg with:
 		if (sd->nchg < -6 || sd->nchg >= 12) {

With u8, negative values won't work.

Signed-off-by: Hans Petter Selasky <hselasky@c2i.net>
Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>