spi: dw: move to SPI core message handling

This patch removes a lot of duplicate code since SPI core provides a nice
message handling.

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

diff --cc drivers/spi/spi-dw-mid.c
Simple merge

diff --cc drivers/spi/spi-dw.c
Simple merge

diff --cc drivers/spi/spi-dw.h
Simple merge