sctp: Do no select unconfirmed transports for retransmissions

An unconfirmed transport is one that we have not been
able to reach since the beginning.  There is no point in
trying to retrasnmit data on those transports.  Also, the
specification forbids it due to security issues.

Reported-by: Frank Schuster <frank.schuster01@web.de>

Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>