irda/irnet: use noop_llseek

There may be applications trying to seek
on the irnet character device, so we should
use noop_llseek to avoid returning an error
when the default llseek changes to no_llseek.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Samuel Ortiz <samuel@sortiz.org>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>