[SCSI] 3w-9xxx: scsi_dma_unmap fix

This patch fixes the following regression the occurred during the
scsi_dma_map()/unmap() changes:

3w-9xxx 0001:45:00.0: 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>