s390/dma: provide proper ARCH_ZONE_DMA_BITS value

On s390 ZONE_DMA is up to 2G, i.e. ARCH_ZONE_DMA_BITS should be 31 bits.
The current value is 24 and makes __dma_direct_alloc_pages() take a
wrong turn first (but __dma_direct_alloc_pages() recovers then).

Let's correct ARCH_ZONE_DMA_BITS value and avoid wrong turns.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Petr Tesarik <ptesarik@suse.cz>
Fixes: c61e9637

 ("dma-direct: add support for allocation from ZONE_DMA and ZONE_DMA32")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>