hotfix: hardcode PLMN selection in the UE to value 1 (was 2)

Fix the assertion:
Assertion ((instance_p->mnc_digit_length[ue_desc_p->selected_plmn_identity] == 3) || (instance_p->mnc_digit_length[ue_desc_p->selected_plmn_identity] == 2)) failed!
In s1ap_eNB_handle_nas_first_req() /home/ubuntu/tmp/openair3/S1AP/s1ap_eNB_nas_procedures.c:243

With this hotfix the UE selects the first PLMN sent by the eNodeB.

The UE has to select the correct PLMN from those received in SIB1. This
will be done with a later merge request.