media: dvb: Remove write permission of other users to debug-fs

There is no need for the dvb debugfs nodes to be world-writable.
Change them to be read-only for other users.

CRs-Fixed: 454805
Change-Id: I56340d5c0431206d81d44c835d558dbb48b24b63
Signed-off-by: Hamad Kadmany <hkadmany@codeaurora.org>