spi: dw: Parameterize the DMA Rx/Tx burst length

It isn't good to have numeric literals in the code especially if there
are multiple of them and they are related. Let's replace the Tx and Rx
burst level literals with the corresponding constants.

Co-developed-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Co-developed-by: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Signed-off-by: Georgy Vlasov <Georgy.Vlasov@baikalelectronics.ru>
Signed-off-by: Ramil Zaripov <Ramil.Zaripov@baikalelectronics.ru>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Alexey Malahov <Alexey.Malahov@baikalelectronics.ru>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Arnd Bergmann <arnd@arndb.de>
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/20200529131205.31838-7-Sergey.Semin@baikalelectronics.ru


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