[media] tvaudio: use V4L2_TUNER_MODE_SAP for TDA985x SAP

As V4L2_TUNER_MODE_SAP == V4L2_TUNER_MODE_LANG2, we make
V4L2_TUNER_MODE_LANG1 equal to V4L2_TUNER_MODE_STEREO.

Signed-off-by: Daniel Glöckner <daniel-gl@gmx.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>