spi: dw-dma: Remove DMA Tx-desc passing around

It's pointless to pass the Rx and Tx transfers DMA Tx-descriptors, since
they are used in the Tx/Rx submit method only. Instead just return the
submission status from these methods. This alteration will make the code
less complex.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112322.24585-8-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Mark Brown <broonie@kernel.org>