[ISDN] capidrv: address two longstanding warnings

* change #warning to a code comment

* add comment and special ifdef'd 64-bit code for a situation where
  we must store a pointer into a CAPI field 'Data', which is fixed by
  the interface at 32 bits.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Acked-by: Karsten Keil <kkeil@suse.de>