[media] adv7170: drop redundant ret local

Simplifies return value logic of adv7170_set_fmt().

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>