media: dvb: Fix stopping of DVR thread

kernel-thread can be stopped either externally using
kthread_stop or the thread function exits by itself, only
one of the approaches should be used.

When kernel thread-function exits the task is cleaned up,
calling kthread_stop afterwords causes failure.

Change-Id: I3eb36e85fa6a650ebdbf2b34dd2ca434f34f28e6
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>