[media] smiapp: Constify the regs argument to smiapp_write_8s()

The data may now be const as well.

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>