media: vimc: Constify vimc_ent_type structures

The vimc_ent_type structure contains static pointers to functions, and
no other information that need to be modified after initialization. Make
them const to avoid the risk of arbitrary code execution following an
overflow that would overwrite the structure's contents.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>