[SCSI] gdth: don't map zero-length requests

Don't map zero-length requests in gdth, zome architectures don't like
that in their dma mapping routines.

[ I'm pretty sure Jens posted this before, but for some reason it got
  forgotten  --hch ]
Signed-off-by: NJames Bottomley <James.Bottomley@SteelEye.com>

drivers/scsi/gdth.c

@@ -2816,7 +2816,7 @@ static int gdth_fill_cache_cmd(int hanum,Scsi_Cmnd *scp,ushort hdrive)
             }
 #endif
 
-        } else {
+        } else if (scp->request_bufflen) {
             scp->SCp.Status = GDTH_MAP_SINGLE;
             scp->SCp.Message = (read_write == 1 ? 
                 PCI_DMA_TODEVICE : PCI_DMA_FROMDEVICE);