media: ti: cal: Constify struct media_entity_operations

'struct media_entity_operations' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  20694	   1394	     32	  22120	   5668	drivers/media/platform/ti/cal/cal-camerarx.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  20726	   1362	     32	  22120	   5668	drivers/media/platform/ti/cal/cal-camerarx.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>