dmaengine: dw-edma: Drop dma_slave_config.direction field usage

dmaengine: dw-edma: Drop dma_slave_config.direction field usage

The dma_slave_config.direction field usage in the DW eDMA driver was
introduced by bd96f1b2f43a ("dmaengine: dw-edma: support local dma device
transfer semantics"). Mainly the change introduced there was correct
(indeed DEV_TO_MEM means using RD-channel and MEM_TO_DEV - WR-channel for
the case of having eDMA accessed locally from CPU/Application side), but
providing an additional MEM_TO_MEM/DEV_TO_DEV-based semantics was quite
redundant if not to say potentially harmful (when it comes to removing the
denoted field). First of all since the dma_slave_config.direction field has
been marked as obsolete (see [1] and the struct dma_slave_config [2]) and
will be discarded in future, using it especially in a non-standard way is
discouraged. Secondly in accordance with the commit denoted above the
default dw_edma_device_transfer() semantics has been changed despite what
its message said. So claiming that the method was left backward compatible
was wrong.

Fix the problems denoted above and simplify the dw_edma_device_transfer()
method by dropping the parsing of the DMA-channel direction field. Instead
of having that implicit dma_slave_config.direction field semantic, use the
recently added DW_EDMA_CHIP_LOCAL flag to distinguish between the local and
remote DW eDMA setups thus preserving support for both cases. Add an ASCII
figure to clarify the situation.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/driver-api/dmaengine/provider.rst?id=v5.18#n478
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/dmaengine.h?id=v5.18#n389

[bhelgaas: convert references to specific URLs]
Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20220524152159.2370739-6-Frank.Li@nxp.com
Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-By: Vinod Koul <vkoul@kernel.org>