'struct llc_conn_state_trans' are 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
  13923	  10896	     32	  24851	   6113	net/llc/llc_c_st.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  21859	   3328	      0	  25187	   6263	net/llc/llc_c_st.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/87cda89e4c9414e71d1a54bb1eb491b0e7f70375.1720973029.git.christophe.jaillet@wanadoo.frSigned-off-by: Jakub Kicinski <kuba@kernel.org>