dmaengine: tegra-apb: Only save channel state for those in use

Currently the tegra-apb DMA driver suspend/resume helpers, save and
restore the registers for all channels regardless of whether they are
in use or not. Change this so that only channels that have been
allocated and configured are saved and restored.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>