[media] em28xx: fix VBI handling logic

When both VBI and video are streaming, and video stream is stopped,
a subsequent trial to restart it will fail, because S_FMT will
return -EBUSY.

That prevents applications like zvbi to work properly.

Please notice that, while this fix it fully for zvbi, the
best is to get rid of streaming_users and res_get logic as a hole.

However, this single-line patch is better to be merged at -stable.

Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>