s390/setup: remove obsolete #ifdef

The #ifdef CONFIG_DMA_API_DEBUG check in reserve_kernel() is no longer
needed, since commit ea535e41

 ("dma-debug: switch check from _text
to _stext") changed the logic in lib/dma-debug.c, so remove it.

Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>