spi: dw-mid: split rx and tx callbacks when DMA

Currently driver wouldn't work properly if user asked for simplex transfer. The
patch separates DMA rx and tx callbacks and finishes transfer correctly in any
case.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>