dmaengine: tegra: Fix uninitialized variable usage

Initialize slave_bw in dma_prep*() functions as the parameter is not
set for DMA_MEM_TO_MEM case in get_transfer_param(). Though the case
may never occur, initializing it avoids warning from certain static
checkers

Fixes: ee170280

 ("dmaengine: tegra: Add tegra gpcdma driver")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Link: https://lore.kernel.org/r/20220426101913.43335-2-akhilrajeev@nvidia.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>