media: visl,vidtv: Set parameters permissions to 0444

This avoids confusion with default values and lets userspace
programs get the modules parameters values at run time.

This can be useful when setting up a test suite.

Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Signed-off-by: Detlev Casanova <detlev.casanova@collabora.com>