Changes

s1gw: fix passing swapped {MME,ENB}_UE_S1AP_ID

This commit fixes a regression introduced in 64db5053
and makes the following TCs green again:

* TC_e_rab_modify_ind_cnf[_multi]
* TC_e_rab_modify_req_rsp[_multi]

Change-Id: I9b19a82234f547a9e44c9c034492ee4b21892818
Fixes: 64db5053 ("s1gw: ConnHdlr: allow passing {MME,ENB}-UE-S1AP-ID")