[media] DVB: dvb_frontend: fix delayed thread exit

There are some issues and miss-behaves at the dvb fe thread:

1) dvb_shutdown_timeout should be dvb_shutdown_timeout * HZ
   instead of (dvb_shutdown_timeout * HZ + 1);

2) add a memory barrier to warrant that all CPU's will consider the
   new value for release_jiffies;

3) wake up dvb thread also when fepriv->exit == DVB_FE_NO_EXIT.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>