nat: Store the msc_con in the data of the bsc_fd

* Stop using nat->msc_con in the read_cb but use the data.
diff --git a/openbsc/src/bsc_msc.c b/openbsc/src/bsc_msc.c
index 42b4c20..c85bf5b 100644
--- a/openbsc/src/bsc_msc.c
+++ b/openbsc/src/bsc_msc.c
@@ -47,6 +47,7 @@
 	fd->when = 0;
 
 	con->is_connected = 0;
+	con->first_contact = 0;
 	con->connection_loss(con);
 }
 
@@ -267,4 +268,4 @@
 	msgb_l16tv_put(msg, strlen(token) + 1,
 			IPAC_IDTAG_UNITNAME, (u_int8_t *) token);
 	return msg;
-}
\ No newline at end of file
+}