[media] ml86v7667: fix compiler warning

build/media_build/v4l/ml86v7667.c: In function 'ml86v7667_s_ctrl':
build/media_build/v4l/ml86v7667.c:120:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret = 0;
        ^

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Cc: Vladimir Barinov <source@cogentembedded.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>