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

m3ua: Fix fsm dispatch failure not generating tx ERR msg to peer

Recent commit reworked those code paths to return an error type instead
of an err message, but wrongly left the negative error of
osmo_fsm_inst_dispatch() code path as is, without converting it to a
positive m3ua error code value, which ended up in an ERR msg not being
generated.

Fixes: 82c1d2c8c6699349258869bce690293b590a6b0d
Change-Id: I16042c97453854c78e55bfe6bd6925506eae82c5