[SCSI] pmcraid: fix to avoid twice scsi_dma_unmap for a command

For a particular driver error condition, driver was doing double
scsi_dma_unmaps. Driver was calling scsi_dma_unmap in
pmcraid_error_handler and return 0. This pmcraid_error_handler is called
by pmcraid_io_done which will do scsi_dma_unmap again when it has
return 0 from pmcraid_error_handler.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>