The command syntax doesn't seem right. I know that you used what's in the Wiki, but it appears that it needs to be corrected.
I imagine the intended syntax was supposed to be
mtd -write /tmp/owrt.bin -d linux
Though, as mtd can do damage if done incorrectly, please wait for confirmation form someone who knows Linux.
Edit: I take that back. It appears that Asus actually include mtd-write, and that the syntax you entered is correct for Asus firmware.
Though, can you confirm that your router is running Asus firmware and not a third-party firmware?