"struct vcap_operations" are not modified in these drivers.

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

In order to do it, "struct vcap_control" also needs to be adjusted to this
new const qualifier.

As an example, on a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  15176	   1094	     16	  16286	   3f9e	drivers/net/ethernet/microchip/lan966x/lan966x_vcap_impl.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  15268	    998	     16	  16282	   3f9a	drivers/net/ethernet/microchip/lan966x/lan966x_vcap_impl.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Link: https://lore.kernel.org/r/d8e76094d2e98ebb5bfc8205799b3a9db0b46220.1718524644.git.christophe.jaillet@wanadoo.frSigned-off-by: Paolo Abeni <pabeni@redhat.com>