[NETFILTER]: xt_sctp: fix --chunk-types matching

xt_sctp uses an incorrect header offset when --chunk-types is used.

Signed-off-by: Jorge Matias <jorge.matias@motorola.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>