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.

openbsc/src/libmsc/mncc_sock.c

@@ -239,7 +239,6 @@ static int mncc_sock_accept(struct bsc_fd *bfd, unsigned int flags)
 		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;
 	}