isdn/capi: Return proper errnos on module init.

cdebug_init() is called from kcapi_init() which is module
initialization function, so it must return negative values on errors.

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>