x86: fix dma_mapping_error for 32bit x86, cleanup

This removes ifdef CONFIG_X86_64 in dma_mapping_error():

1) Xen people plan to use swiotlb on X86_32 for Dom0 support. swiotlb
   uses ops->mapping_error so X86_32 also needs to check
   ops->mapping_error.

2) Removing #ifdef hack is almost always a good thing.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>