Remove 'isdn_* is defined but unused' warnings

Move the #ifdef guard above these functions to remove the warnings.

Signed-off-by: Charlie Shepherd <masterdriverz@gentoo.org>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>