[media] em28xx: fix incorrect s_ctrl error code and wrong call to res_free

Calling subdevs to handle s_ctrl returned a non-zero return code even if
everything went fine.

Calling STREAMOFF if no STREAMON happened earlier would hit a BUG_ON
in res_free.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>