media: mark entity-intf links as IMMUTABLE

Currently links between entities and an interface are just marked as
ENABLED. But (at least today) these links cannot be disabled by userspace
or the driver, so they should also be marked as IMMUTABLE.

It might become possible that drivers can disable such links (if for some
reason the device node cannot be used), so we might need to add a new link
flag at some point to mark interface links that can be changed by the driver
but not by userspace.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>