[media] pvrusb2: correctly handling failed thread run

Since kthread_run returns -ENOMEM if failed,
it needs to be checked whether it is error, not whether it is null.

Signed-off-by: Insu Yun <wuninsu@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>