[LLC]: llc_mac_hdr_init const arguments

Cleanup of LLC.  llc_mac_hdr_init can take constant arguments,
and it is defined twice once in llc_output.h that is otherwise unused.

Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Acked-by: Arnaldo Carvalho de Melo <acme@mandriva.com>
Signed-off-by: David S. Miller <davem@davemloft.net>