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>