spi: dw: Avoid useless assignments in generic DMA setup

Generic DMA setup doesn't rely on certain type of DMA controller and thus
shouldn't use Intel Medfield settings, although it's harmless in this case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200507115449.8093-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>