isdn: icn: remove a #warning

The icn driver currently produces an unconditional #warning whenever
we build it, introduced by Karsten Keil back in 2003:

 #warning TODO test headroom or use skb->nb to flag ACK

Karsten's original commit (from BitKeeper) contains this description:

    - here are lot of bugs left, so ISDN is not stable yet but
      I think it's really time to fix it, even if it need some cycles
      to get it right (normally I'm only send patches if it works 100% for
      me).
    - I add some additional #warnings to address places which need fixing
      (I hope that some of the other ISDN developer jump in)

Apparently this has not happened, and it is unlikely that it ever will,
given that the driver doesn't seem to work. No substantial bug fixes
other than janitorial cleanups have happened in the driver since then,
and I see no indication that anyone who patched it had the hardware.

We should probably either remove the driver, or remove all of i4l,
but for now, this shuts up the distracting #warning by turning it
into a comment.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: http://git.meleeweb.net/linux.git/commit/?id=b0deac0886b0056765afd149e9834373b38e096b


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>