spi: dw-dma: Set DMA Level registers on init

spi: dw-dma: Set DMA Level registers on init

Indeed the registers content doesn't get cleared when the SPI controller
is disabled and enabled. Max burst lengths aren't changed since the Rx and
Tx DMA channels are requested on init stage and are kept acquired until
the device is removed. Obviously SPI controller FIFO depth can't be
changed. Due to all of that we can safely move the DMA Transmit and
Receive data level registers initialization to the SPI controller DMA init
stage (when the SPI controller is being probed) instead of doing it for
each SPI transfer when dma_setup is called. This shall speed the DMA-based
SPI transfer initialization up a bit, particularly if the APB bus is
relatively slow.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112322.24585-2-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>