[nat] Lookup by BSC Connection otherwise the point of reassigning the is defeated

When sending a MSG to the MSC try to find the to be used "src" reference
by comparing the reference on the BSC and the BSC connection. Only this
tuple needs to be unique.
Actually only when looking at the SRC REF we need to compare the BSC as the
dest reference should be unique but we are just making the check a bit stronger
to make it look symmetric.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 00dcc88..eaa9d30 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -524,17 +524,17 @@
 		case SCCP_MSG_TYPE_CR:
 			if (create_sccp_src_ref(bsc, msg, parsed) != 0)
 				goto exit2;
-			con = patch_sccp_src_ref_to_msc(msg, parsed, nat);
+			con = patch_sccp_src_ref_to_msc(msg, parsed, bsc);
 			break;
 		case SCCP_MSG_TYPE_RLSD:
 		case SCCP_MSG_TYPE_CREF:
 		case SCCP_MSG_TYPE_DT1:
 		case SCCP_MSG_TYPE_CC:
 		case SCCP_MSG_TYPE_IT:
-			con = patch_sccp_src_ref_to_msc(msg, parsed, nat);
+			con = patch_sccp_src_ref_to_msc(msg, parsed, bsc);
 			break;
 		case SCCP_MSG_TYPE_RLC:
-			con = patch_sccp_src_ref_to_msc(msg, parsed, nat);
+			con = patch_sccp_src_ref_to_msc(msg, parsed, bsc);
 			remove_sccp_src_ref(bsc, msg, parsed);
 			break;
 		case SCCP_MSG_TYPE_UDT: