sctp: remove an if() that is always true

As noticed by Xin Long, the if() here is always true as PMTU can never
be 0.

Reported-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>