spi: dw-dma: Detach DMA transfer into a dedicated method

spi: dw-dma: Detach DMA transfer into a dedicated method

In order to add an alternative method of DMA-based SPI transfer first we
need to detach the currently available one from the common code. Here we
move the normal DMA-based SPI transfer execution functionality into a
dedicated method. It will be utilized if either the DMA engine supports
an unlimited number SG entries or Tx-only SPI transfer is requested. But
currently just use it for any SPI transfer.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112322.24585-9-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>