spi: Fix handling of cs_change in core implementation
The core implementation of cs_change didn't follow the documentation
which says that cs_change in the middle of the transfer means to briefly
deassert chip select, instead it followed buggy drivers which change the
polarity of chip select. Use a delay of 10us between deassert and
reassert simply from pulling numbers out of a hat.
Reported-by: Gerhard Sittig <gsi@denx.de> Signed-off-by: Mark Brown <broonie@linaro.org>