dmaengine: at_hdmac: check that each sg data length is non-null

commit c456797681db814f4f5b36909e8e94047bf53d9c upstream.

Avoid the construction of a malformed DMA request sent to
the DMA controller.
Log message is for debug only because this condition is unlikely to
append and may only trigger at driver development time.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>