Make ISDN CAPI use seq_list_xxx helpers

The similar code exists here and is called capi_driver_get_idx().  Use generic
helpers now and remember to convert list_head to struct capi_driver in .show
callback.

Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>