mtd: spinand: Constify struct nand_ecc_engine_ops

'struct nand_ecc_engine_ops' are not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

Update the prototype of mxic_ecc_get_pipelined_ops() accordingly.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  16709	   1374	     16	  18099	   46b3	drivers/mtd/nand/ecc-mxic.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  16789	   1294	     16	  18099	   46b3	drivers/mtd/nand/ecc-mxic.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/72597e9de2320a4109be2112e696399592edacd4.1729271136.git.christophe.jaillet@wanadoo.fr