[media] smiapp: Unify enforced and need-based 8-bit read

Unify enforced 8-bit read access with that based on actual need.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>