[media] gspca: passing wrong length parameter to reg_w()

This looks like a cut an paste error.  This is a two byte array but we
use 8 as a length parameter.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>