llc: Make llc_sap_action_t function pointer arrays const

It's better when function pointer arrays aren't modifiable.

Net change:

$ size net/llc/built-in.o.*
   text	   data	    bss	    dec	    hex	filename
  61193	  12758	   1344	  75295	  1261f	net/llc/built-in.o.new
  47113	  27030	   1344	  75487	  126df	net/llc/built-in.o.old

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: David S. Miller <davem@davemloft.net>