[SCSI] ibmvscsi: fix DMA API misuse

ibmvscsi uses dma_unmap_single() for buffers mapped via
dma_map_sg(). It works however it's the API violation. The DMA debug
facility complains about it:

http://marc.info/?l=linux-scsi&m=127018555013151&w=2



Reported-by: Sachin Sant <sachinp@in.ibm.com>
Tested-by: Sachin Sant <sachinp@in.ibm.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>