net: dsa: hellcreek: Constify struct devlink_region_ops and struct hellcreek_fdb_entry

net: dsa: hellcreek: Constify struct devlink_region_ops and struct hellcreek_fdb_entry

'struct devlink_region_ops' and 'struct hellcreek_fdb_entry' 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:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  55320	  19216	    320	  74856	  12468	drivers/net/dsa/hirschmann/hellcreek.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  55960	  18576	    320	  74856	  12468	drivers/net/dsa/hirschmann/hellcreek.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://patch.msgid.link/2f7e8dc30db18bade94999ac7ce79f333342e979.1751231174.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>