snap: use const for descriptor

Protocols should be able to use constant value for the descriptor.
Minor whitespace cleanup as well

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