um: fix error return code in slip_open()

Fix to return a negative error code from the error handling case instead
of 0, as done elsewhere in this function.

Fixes: a3c77c67a4 ("[PATCH] uml: slirp and slip driver cleanups and fixes")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-By: anton.ivanov@cambridgegreys.com
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/drivers/slip_user.c

@@ -145,7 +145,8 @@ static int slip_open(void *data)
 	}
 	sfd = err;
 
-	if (set_up_tty(sfd))
+	err = set_up_tty(sfd);
+	if (err)
 		goto out_close2;
 
 	pri->slave = sfd;