dmaengine: at_hdmac: fix comment in atc_prep_slave_sg()

commit c618a9be0e8c0f36baee2560860a0118a428fb26 upstream.

s/dma_memcpy/slave_sg/ and it is sg length that we are
talking about.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>