scsi: g_NCR5380: Fix PDMA transfer size

generic_NCR5380_dma_xfer_len() incorrectly uses cmd->transfersize which
causes rescan-scsi-bus and CD-ROM access to hang the system.  Use
cmd->SCp.this_residual instead, like other NCR5380 drivers.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>