spi/tegra20-slink: Move first transfer preparation to prepare_message

This is more idiomatic for the factored out message processing and gives a
small simplification of the code since we always set the per-transfer
parameters in the same fashion.

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

diff --cc drivers/spi/spi-tegra20-slink.c
Simple merge