Add DMA workaround for chips that do not support full 64-bit DMA
addresses.

5714, 5715, and 5780 chips only support DMA addresses less than 40
bits. On 64-bit systems with IOMMU, set the dma_mask to 40-bit so
that pci_map_xxx() calls will map the DMA address below 40 bits if
necessary. On 64-bit systems without IOMMU, set the dma_mask to
64-bit and check for DMA addresses exceeding the limit in
tg3_start_xmit().

5788 only supports 32-bit DMA so need to set the mask appropriately
also.

Thanks to Chris Elmquist at SGI for reporting and helping to debug
the problem on 5714.

Thanks to David Miller for explaining the HIGHMEM and DMA stuff.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>