mncc: Do not permanently disable the listening socket in case of error

When we have no other MNCC connection but the registeration of the
new fd is failing we should not disable reading from the listen_fd
for ever as the situation might not be permanent.
diff --git a/openbsc/src/libmsc/mncc_sock.c b/openbsc/src/libmsc/mncc_sock.c
index 576ed01..d709a72 100644
--- a/openbsc/src/libmsc/mncc_sock.c
+++ b/openbsc/src/libmsc/mncc_sock.c
@@ -239,7 +239,6 @@
 		LOGP(DMNCC, LOGL_ERROR, "Failed to register new connection fd\n");
 		close(conn_bfd->fd);
 		conn_bfd->fd = -1;
-		state->listen_bfd.when |= ~BSC_FD_READ;
 		return -1;
 	}