x86, mm: Use MAX_DMA_PFN for ZONE_DMA on 32-bit

Use MAX_DMA_PFN which represents the 16 MB ISA DMA limit on
32-bit x86 just like we do on 64-bit.

Acked-by: Tejun Heo <tj@kernel.org>
Acked-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
Link: http://lkml.kernel.org/r/1320155902-10424-1-git-send-email-penberg@kernel.org


Signed-off-by: Ingo Molnar <mingo@elte.hu>