[media] cxd2820r_t2: Fix a warning: stream_id is unsigned

drivers/media/dvb-frontends/cxd2820r_t2.c: In function 'cxd2820r_set_frontend_t2':
drivers/media/dvb-frontends/cxd2820r_t2.c:128:2: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>