scsi: aha1542: Fix zeroday __udivdi3 warning

dma_addr_t can be u64 on pae systems but isa_virt_to_bus only ever
returns unsigned long (because an ISA physical address can only be 24
bits). Cast to unsigned long to avoid division.

Fixes: 1794ef2b

 ("scsi: aha1542: convert to DMA mapping API")
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>