spi: tegra: slink: make local symbols static

Neither tegra20_spi_cdata nor tegra30_spi_cdata are used outside this
file so they can, and should, be static.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>