x86, 32-bit: Fix swiotlb boot crash

Ingo Molnar reported this boot crash:

[    8.655620] pata_amd 0000:00:06.0: version 0.4.1
[    8.660286] BUG: unable to handle kernel NULL pointer dereference at 00000034
[    8.663572] IP: [<c100617b>] dma_supported+0x3b/0xa4
[    8.663572] *pde = 00000000

Initialize dma_ops properly in the 32-bit case.

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