net: dsa: mv88e6xxx: Constify struct devlink_region_ops and struct mv88e6xxx_region

net: dsa: mv88e6xxx: Constify struct devlink_region_ops and struct mv88e6xxx_region

'struct devlink_region_ops' and 'struct mv88e6xxx_region' are not modified
in this driver.

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

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  18076	   6496	     64	  24636	   603c	drivers/net/dsa/mv88e6xxx/devlink.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  18652	   5920	     64	  24636	   603c	drivers/net/dsa/mv88e6xxx/devlink.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/46040062161dda211580002f950a6d60433243dc.1751200453.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>