spi: dw-dma: Move DMA transfers submission to the channels prep methods

Indeed we can freely move the dmaengine_submit() method invocation and the
Tx and Rx busy flag setting into the DMA Tx/Rx prepare methods. Since the
Tx/Rx preparation method is now mainly used for the DMA transfers
submission, here we suggest to rename it to have the _submit_{r,t}x suffix
instead.

By having this alteration applied first we implement another code
preparation before adding the one-by-one DMA SG entries transmission,
second we now have the dma_async_tx_descriptor descriptor used locally
only in the new DMA transfers submission methods (this will be cleaned up
a bit later), third we make the generic transfer method more readable,
where now the functionality of submission, execution and wait procedures
is transparently split up instead of having a preparation, intermixed
submission/execution and wait procedures.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112322.24585-6-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>

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