V4L/DVB: hdpvr: Fixes probing function

In the hdpvr_probe () function, when an error occurs while probing the device,
the workqueue created by the create_single_thread () call is not properly
destroyed.

Signed-off-by: Perceval Anichini <perceval@trilogic.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>