Remove virt_to_bus/bus_to_virt code from dpt_i2o, and use
dma_alloc_coherent() / dma_free_coherent().

This is in preparation of 64-bit support, dma_alloc_coherent()
can allocate memory in the lower 32 bits of physical memory
which is needed because the HBA only supports message blocks under 4GB

This code is based in part on the unofficial adaptec 64-bit
dpt_i2o driver update that I got from Mark Salyzyn at Adaptec.
Signed-off-by: Miquel van Smoorenburg <miquels@cistron.nl>
Acked-by: Mark Salyzyn <Mark_Salyzyn@adaptec.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>