[SCSI] buslogic: Added check for DMA mapping errors

Added check for DMA mapping errors for request sense data
buffer. Checking for mapping error can avoid potential wild
writes. This patch was prompted by the warning from
dma_unmap when kernel is compiled with CONFIG_DMA_API_DEBUG.

Signed-off-by: Khalid Aziz <khalid.aziz@oracle.com>
Tested-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>