'struct llc_sap_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
    339	    456	     24	    819	    333	net/llc/llc_s_st.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
    683	    144	      0	    827	    33b	net/llc/llc_s_st.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/9d17587639195ee94b74ff06a11ef97d1833ee52.1720973710.git.christophe.jaillet@wanadoo.frSigned-off-by: Jakub Kicinski <kuba@kernel.org>