spi/s3c64xx: Factor transfer start out of enable/disable_cs()

The hardware level /CS handling is tied to the start of the data path so
is rolled into the same function as we use to manipulate GPIO /CS. In
order to support factoring out the /CS handling into the core separate the
two and explicitly start transfers separately to the /CS handling.

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