[media] ml86v7667: fix compile warning: 'ret' set but not used

media_build/v4l/ml86v7667.c: In function 'ml86v7667_s_ctrl':
media_build/v4l/ml86v7667.c:120:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
And indeed, ret is set but not used. Let's actually return the error
code.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>