dmaengine: tegra-apb: Remove unused variables

The callback and callback_param members of the tegra_dma_sg_req structure
are never used. The dma-engine structure, dma_async_tx_descriptor, defines
the same members and these are the ones used by the driver. Therefore,
remove the unused versions from the tegra_dma_sg_req structure.

The half_done member of tegra_dma_channel structure is configured but
never used and so remove it.

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