media: dvb: Protect against variable update

DVR device variables may be modified by DVR input
thread which protects against the variables
update using a spin-lock. Such protection is required
also when variables are updated in DVR release function
that is executed upon user-space request.

Change-Id: I5d6a7aeba0a74f2496d7b645321d379cba6fb8b7
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>