spi: dw: Add SPI Tx-done wait method to DMA-based transfer

spi: dw: Add SPI Tx-done wait method to DMA-based transfer

Since DMA transfers are performed asynchronously with actual SPI bus
transfers, then even if DMA transactions are finished it doesn't mean
all data is actually pushed to the SPI bus. Some data might still be
in the controller FIFO. This is specifically true for Tx-only transfers.
In this case if the next SPI transfer is recharged while a tail of the
previous one is still in FIFO, we'll loose that tail data. In order to
fix that problem let's add the wait procedure of the Tx SPI transfer
completion after the DMA transactions are finished.

Fixes: 7063c0d942a1 ("spi/dw_spi: add DMA support")
Co-developed-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Signed-off-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Cc: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Feng Tang <feng.tang@intel.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: devicetree@vger.kernel.org
Link: https://lore.kernel.org/r/20200529131205.31838-5-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>