dmaengine: tegra-apb: Avoid unnecessary channel base address calculation

Everytime a DMA channel register is accessed, the channel base address
is calculated by adding the DMA base address and the channel register
offset. Avoid this calculation and simply calculate the channel base
address once at probe time for each DMA channel.

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