[media] dvb: earth-pt1: stop polling data when no one accesses the device

The driver started a kthread to poll the DMA buffers soon after probing,
which relsuleted in 1000/sec sleeps and wakeups of the thread doing nothing
useful until someone started feeding.
This patch changes the creation and destruction of the kthread depending on the number of users.

Signed-off-by: Akihiro Tsukada <tskd2@yahoo.co.jp>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>