V4L/DVB (8022): saa7134: fix race between opening and closing the device

decrementing dev->empress_users should be done as last action of ts_release,
because it sleeps and write access to dev->empress_started is not protected
in any way
(additionally closing thread could mute audio after opening thread unmuted it)

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>