Changes

sccp: Use protoClass 0 to transmit SSA

Recent commit added the possibility to set proto Class 0 vs 1 based on
Sequence Control parameter present in UNITDATA.req, but forgot to extend
it to the special code path used tro transmit SSA.

As a result, libosmo-sigtran started transmitting SSA with protoClass 1
instead of previous 0. This is actually not bad, but there's no need to
use protocClass1 here since we don't care about other messages being
sent together with this one, so protoClass0 is better fit here.

Fixes: 0061e8d0bcba3b0ed5ea255588619627d0975380
Change-Id: I906094a14be670e665ed8ffd0383dd8da690cb9a