bugfix: fix PCDP inst (again)

This commit follows 7d9945e8.

lte-softmodem UE in S1 and lte-softmodem UE in noS1 modes behave
differently here.

This commit fixes the issue for thoses cases.

Other cases (eNB S1, eNB noS1, oaisim S1/noS1) have to be checked.