[media] au0828: fix case where STREAMOFF being called on stopped stream causes BUG()

We weren't checking whether the resource was in use before calling
res_free(), so applications which called STREAMOFF on a v4l2 device that
wasn't already streaming would cause a BUG() to be hit (MythTV).

Reported-by: Larry Finger <larry.finger@lwfinger.net>
Reported-by: Jay Harbeston <jharbestonus@gmail.com>
Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>