i2c: tegra: use dmaengine_get_dma_device() for DMA buffer allocation

Use dmaengine_get_dma_device() to obtain the correct struct device
pointer for dma_alloc_coherent() instead of directly dereferencing
chan->device->dev.

The dmaengine_get_dma_device() helper checks whether the DMA channel
has a per-channel DMA device (chan->dev->chan_dma_dev) and returns it
when available, falling back to the controller device otherwise. On
platforms where the DMA controller sits behind an IOMMU with
per-channel IOVA spaces (e.g. Tegra264 GPC DMA), the per-channel
device carries the correct DMA mapping context. Using the controller
device directly would allocate DMA buffers against the wrong IOMMU
domain, leading to SMMU faults at runtime.

On platforms without per-channel DMA devices the helper returns the
same pointer as before, so there is no change in behavior for existing
hardware.

Assisted-by: Cursor:claude-4.6-opus
Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260518114013.62065-2-akhilrajeev@nvidia.com

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