[SCSI] 3w-xxxx: scsi_dma_unmap fix

This patch fixes the following regression that occurred during the
scsi_dma_map()/unmap()
changes when compiling with CONFIG_DMA_API_DEBUG=y :

WARNING: at lib/dma-debug.c:496 check_unmap+0x142/0x542()
Hardware name:
3w-xxxx 0000:02:02.0: DMA-API: device driver tries to free DMA memory
it has not allocated [device address=0x0000000000000000] [size=36
bytes]

Signed-off-by: Adam Radford <aradford@gmail.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>