[SCTP] net/sctp/auth.c: make 3 functions static

This patch makes three needlessly global functions static.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>