media: media/dvb: earth-pt3: fix hang-up in a rare case

When a user starts and stops filtering at a demux device too quickly
in a very short interval, the user process hangs in uninterruptible sleep,
due to an inconsistency of kthread status in the driver.
The kthread can be stopped before it starts running its thread function,
but the invocation status was partly managed in the kthread function,
which resulted in a double kthread_stop() of one kthread.

Signed-off-by: Akihiro Tsukada <tskd08@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>