Changes

sctp_proxy: fix wrong operator in connecting/3

We need to update the map, thus the update operation (`=>`) needs
to be used, not the matching (`:=`).  We're lucky that eNBs usually
do not send anything before the MME responds to the S1Setup request;
otherwise the S1GW would crash here due to a mismatch.

Change-Id: I26d8a8da5cce89324a9e1150e4ecdf2084d097c8