dvb: dvb-core: Use kthread instead of workqueue to process DVR input

Existing code uses workqueue for DVR input work which constantly waits
for new data written to the input buffer. This causes the kworker
executing DVR work to be fully occupied by DVR device and does not
allow other drivers using workqueues to use that kworker. To solve
this switched to kernel thread which is solely used for DVR device.

Change-Id: Ica740965aceec63e95eb2be4b3c20e242a8301fe
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>
Signed-off-by: Neha Pandey <nehap@codeaurora.org>