[SCSI] advansys: Use dma mapping for overrun buffer

Convert the call to virt_to_bus() into a call to dma_map_single().  Some
architectures may require different DMA addresses for different devices,
so allocate one overrun buffer per host rather than one for all cards.

Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>