dmaengine: at_hdmac: fix comment in atc_prep_slave_sg()

s/dma_memcpy/slave_sg/ and it is sg length that we are
talking about.
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Stable <stable@vger.kernel.org> [2.6.31+]
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>

drivers/dma/at_hdmac.c

@@ -661,7 +661,7 @@ atc_prep_slave_sg(struct dma_chan *chan, struct scatterlist *sgl,
 			flags);
 
 	if (unlikely(!atslave || !sg_len)) {
-		dev_dbg(chan2dev(chan), "prep_dma_memcpy: length is zero!\n");
+		dev_dbg(chan2dev(chan), "prep_slave_sg: sg length is zero!\n");
 		return NULL;
 	}