snap: handle registration error and compile warning

If this module can't load, it is almost certainly because something else
is already bound to that SAP. So in that case, return the same error code
as other SAP usage, and fail the module load.

Also fixes a compiler warning about printk of non const.

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>