rcar-hpbdma: fixup channel busy check for double plane

The device busy check method is different between
single and double planes.
It will always return "busy" without this patch
if channel used as double plane.

Reviewed-by: Max Filippov <max.filippov@cogentembedded.com>
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>