spi/tegra20-slink: Crude refactoring to use core message parsing

This is a half done conversion with minimal code reorganisation provided
for bisection purposes. A further patch will move the first transfer
preparation into tegra_slink_prepare_message().

The cs_change and udelay handling is removed, these should be
implemented by the framework and in any case are buggy - the two fields
should not be related and the cs_change handling appears to at best only
work the first time it's used in a message.

Signed-off-by: Mark Brown <broonie@linaro.org>
Tested-by: Stephen Warren <swarren@nvidia.com>